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/ui/effects/framebuffer.h

@@ -125,7 +125,6 @@ private:
     
     GLuint fbo_ = 0;
     GLuint colorTexture_ = 0;
-    GLuint depthRbo_ = 0;  // Renderbuffer for depth (optional)
     
     int width_ = 0;
     int height_ = 0;