build(freetype): color-emoji rendering via a cross-platform FreeType backend

The chat "Emoji style: Color" option renders the merged emoji in color (COLR/CPAL
Twemoji) instead of the monochrome NotoEmoji subset. This needs FreeType, which the
default stb_truetype rasterizer can't do for color glyphs.

- Vendor imgui_freetype (matches the bundled 1.92 ImFontLoader API) and embed a 1.4 MB
  COLRv0 Twemoji font (no libpng/harfbuzz needed).
- CMake gains an optional FreeType path: native Linux/macOS use the system FreeType via
  find_package; the mingw-w64 cross-compile has none, so build.sh --win-release now
  cross-builds a minimal static FreeType (scripts/build-freetype-mingw.sh) and passes it
  in. Absent FreeType => graceful monochrome fallback, so no build breaks.
- Typography selects the FreeType loader + the color font (LoadColor) when color emoji is
  on, else the stb loader + mono subset; toggling reloads the atlas. Both the DX11 and
  OpenGL backends already support the 1.92 RGBA dynamic atlas, so color glyphs render.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/embedded/embedded_fonts.cpp.in

@@ -16,3 +16,4 @@ INCBIN(material_icons,  "@CMAKE_SOURCE_DIR@/res/fonts/MaterialIcons-Regular.ttf"
 INCBIN(mdi_pickaxe_subset, "@CMAKE_SOURCE_DIR@/res/fonts/MaterialDesignIcons-Pickaxe-Subset.ttf");
 INCBIN(noto_cjk_subset, "@CMAKE_SOURCE_DIR@/res/fonts/NotoSansCJK-Subset.ttf");
 INCBIN(noto_emoji_subset, "@CMAKE_SOURCE_DIR@/res/fonts/NotoEmoji-Subset.ttf");
+INCBIN(twemoji_color, "@CMAKE_SOURCE_DIR@/res/fonts/TwemojiMozilla-Color.ttf");

src/embedded/embedded_fonts.h

@@ -38,4 +38,8 @@ extern "C" {
 
     extern const unsigned char g_noto_emoji_subset_data[];
     extern const unsigned int  g_noto_emoji_subset_size;
+
+    // Twemoji COLR/CPAL color-emoji font (used only when color emoji is enabled + FreeType is available).
+    extern const unsigned char g_twemoji_color_data[];
+    extern const unsigned int  g_twemoji_color_size;
 }

src/ui/material/typography.cpp

@@ -15,6 +15,11 @@
 #include "../embedded/IconsMaterialDesign.h" // Icon codepoint defines
 #include "../../util/logger.h"
 
+#ifdef DRAGONX_HAVE_FREETYPE
+#include "imgui_internal.h"                   // ImFontAtlasGetFontLoaderForStbTruetype
+#include "misc/freetype/imgui_freetype.h"     // ImGuiFreeType::GetFontLoader + LoadColor flag
+#endif
+
 namespace dragonx {
 namespace ui {
 namespace material {
@@ -125,7 +130,16 @@ bool Typography::load(ImGuiIO& io, float dpiScale)
     float scale = dpiScale * Layout::kFontScale() * Layout::userFontScale();
     DEBUG_LOGF("Typography: Loading Material Design type scale (DPI: %.2f, fontScale: %.2f, userFontScale: %.2f, combined: %.2f)\n",
                dpiScale, Layout::kFontScale(), Layout::userFontScale(), scale);
-    
+
+#ifdef DRAGONX_HAVE_FREETYPE
+    // Choose the atlas font loader BEFORE any font is added: FreeType (required to rasterize COLR
+    // color-emoji glyphs) when color emoji is enabled, else the default stb_truetype loader. Toggling
+    // the setting + reload() flips this cleanly.
+    io.Fonts->SetFontLoader(color_emoji_ ? ImGuiFreeType::GetFontLoader()
+                                         : ImFontAtlasGetFontLoaderForStbTruetype());
+    DEBUG_LOGF("Typography: font loader = %s\n", color_emoji_ ? "FreeType (color emoji)" : "stb_truetype");
+#endif
+
     // For ImGui, we need to load fonts at specific pixel sizes.
     // Font sizes come from Layout:: accessors (backed by UISchema JSON)
     
@@ -335,9 +349,22 @@ ImFont* Typography::loadFont(ImGuiIO& io, int weight, float size, const char* na
         };
         bool wantEmoji = false;
         for (const char* n : kEmojiFonts) if (strcmp(name, n) == 0) { wantEmoji = true; break; }
-        if (wantEmoji && g_noto_emoji_subset_size > 0) {
-            void* emojiCopy = IM_ALLOC(g_noto_emoji_subset_size);
-            memcpy(emojiCopy, g_noto_emoji_subset_data, g_noto_emoji_subset_size);
+
+        // Emoji blob: the COLR/CPAL color font (FreeType-rendered) when color emoji is enabled and this
+        // is a FreeType build, else the monochrome subset (default / non-FreeType path).
+        const unsigned char* emojiData = g_noto_emoji_subset_data;
+        unsigned int         emojiSize = g_noto_emoji_subset_size;
+        bool                 colorGlyphs = false;
+#ifdef DRAGONX_HAVE_FREETYPE
+        if (color_emoji_ && g_twemoji_color_size > 0) {
+            emojiData   = g_twemoji_color_data;
+            emojiSize   = g_twemoji_color_size;
+            colorGlyphs = true;
+        }
+#endif
+        if (wantEmoji && emojiSize > 0) {
+            void* emojiCopy = IM_ALLOC(emojiSize);
+            memcpy(emojiCopy, emojiData, emojiSize);
 
             ImFontConfig emojiCfg;
             emojiCfg.FontDataOwnedByAtlas = true;
@@ -346,6 +373,9 @@ ImFont* Typography::loadFont(ImGuiIO& io, int weight, float size, const char* na
             emojiCfg.OversampleV = 1;
             emojiCfg.PixelSnapH = true;
             emojiCfg.GlyphMinAdvanceX = 0;
+#ifdef DRAGONX_HAVE_FREETYPE
+            if (colorGlyphs) emojiCfg.FontLoaderFlags |= ImGuiFreeTypeLoaderFlags_LoadColor;  // render COLR in color
+#endif
             // The base Ubuntu font already owns U+2600–26FF etc.; MergeMode keeps the first-loaded glyph,
             // so its text-style symbols win and only the codepoints it lacks fall through to emoji.
             static const ImWchar emojiRanges[] = {
@@ -355,11 +385,13 @@ ImFont* Typography::loadFont(ImGuiIO& io, int weight, float size, const char* na
                 0,
             };
             emojiCfg.GlyphRanges = emojiRanges;
-            snprintf(emojiCfg.Name, sizeof(emojiCfg.Name), "NotoEmoji %.0fpx (merge)", size);
+            snprintf(emojiCfg.Name, sizeof(emojiCfg.Name), "%s %.0fpx (merge)",
+                     colorGlyphs ? "Twemoji" : "NotoEmoji", size);
 
-            ImFont* emojiMerge = io.Fonts->AddFontFromMemoryTTF(emojiCopy, g_noto_emoji_subset_size, size, &emojiCfg);
+            ImFont* emojiMerge = io.Fonts->AddFontFromMemoryTTF(emojiCopy, emojiSize, size, &emojiCfg);
             if (emojiMerge) {
-                DEBUG_LOGF("Typography: Merged emoji (%u bytes) into %s OK\n", g_noto_emoji_subset_size, name);
+                DEBUG_LOGF("Typography: Merged %s emoji (%u bytes) into %s OK\n",
+                           colorGlyphs ? "color" : "mono", emojiSize, name);
             } else {
                 DEBUG_LOGF("Typography: WARNING — emoji merge FAILED for %s (size=%u)\n", name, size);
             }

src/ui/material/typography.h

@@ -112,6 +112,14 @@ public:
      * @brief Get the current DPI scale
      */
     float getDpiScale() const { return dpiScale_; }
+
+    /**
+     * @brief Select color vs monochrome emoji for the next (re)load. Color needs a FreeType-enabled
+     *        build (DRAGONX_HAVE_FREETYPE); otherwise this is inert and monochrome is always used.
+     *        Set before load()/reload() (App does this from the chat_emoji_color setting).
+     */
+    void setColorEmoji(bool enabled) { color_emoji_ = enabled; }
+    bool colorEmoji() const { return color_emoji_; }
     
     /**
      * @brief Get font for a type style
@@ -261,7 +269,8 @@ private:
     
     bool loaded_ = false;
     float dpiScale_ = 1.0f;
-    
+    bool color_emoji_ = false;   // when true + FreeType present, merge the COLR color-emoji font
+
     // Fonts for each type style
     ImFont* fonts_[15] = {};