perf(gl): drop the unused depth/stencil buffers from window and FBOs

The GL window asked for a 24-bit depth and 8-bit stencil buffer, and
every acrylic Framebuffer attached a DEPTH24_STENCIL8 renderbuffer the
size of its color texture. Nothing uses them: no code enables depth or
stencil tests (the ImGui GL3 backend disables both), every blit and
readback is color-only, and only Framebuffer::clear() also cleared the
depth buffer, which nothing read. The window and every FBO, including
the full-size capture FBO, carried that memory, and the FBO
renderbuffers were reallocated on every resize.

Now:
- the window requests depth 0 and stencil 0 explicitly (SDL3 requests a
  depth buffer by default when the attributes are simply removed);
  alpha 8 stays for the transparent window;
- Framebuffer has no renderbuffer, and clear() clears color only;
- a permanent startup log line,
  "GL default framebuffer: depth=N stencil=N", reports what the driver
  provided; "(no attachment)" marks a failed query, which is how some
  drivers report a buffer that does not exist.

The log shows depth=0 stencil=0 both offscreen and on WSLg X11, where
"Window transparency enabled by compositor" is still logged, with no
framebuffer-incomplete errors. Screenshot sweeps at 1280x720 and
2560x1440, and a glass run with ui_opacity 0.6 and blur on (so panels go
through the FBO blur), match the previous build within noise. The
driver-side memory saving itself was not measured. DX11 is unaffected:
its swap chain has no depth buffer.

Not verifiable here: macOS (an NSOpenGL 3.2 core context with a
transparent window).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/main.cpp

@@ -1233,8 +1233,9 @@ int main(int argc, char* argv[])
 
     // Create window with graphics context
     SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
-    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
-    SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8);
+    // Nothing in the app uses depth/stencil (ImGui + acrylic are color-only): request none.
+    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 0);
+    SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 0);
     SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);
 
     SDL_WindowFlags window_flags = (SDL_WindowFlags)(SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE | SDL_WINDOW_HIGH_PIXEL_DENSITY | SDL_WINDOW_TRANSPARENT);
@@ -1268,6 +1269,11 @@ int main(int argc, char* argv[])
     }
 
     SDL_GL_MakeCurrent(window, gl_context);
+    { int d = -1, s = -1;
+      const bool okd = SDL_GL_GetAttribute(SDL_GL_DEPTH_SIZE, &d);
+      const bool oks = SDL_GL_GetAttribute(SDL_GL_STENCIL_SIZE, &s);
+      DEBUG_LOGF("GL default framebuffer: depth=%d%s stencil=%d%s\n",
+                 d, okd ? "" : " (no attachment)", s, oks ? "" : " (no attachment)"); }
     SDL_GL_SetSwapInterval(1); // Enable vsync
 
     bool backdrop_active = false;