docs(gal-test): Add README and document DrawBitmap limitation

- Add comprehensive README documenting all 28 GAL test scenarios
- Document known limitation: DrawBitmap uses legacy OpenGL immediate
  mode (glBegin/glEnd) incompatible with shader-based test harness
- Add shader accessor for DrawBitmap workaround attempt
- Clean up debug code from bitmap scenario

The DrawBitmap limitation is acceptable because:
- It's primarily used for reference images in schematics
- WebGL port will need its own bitmap rendering implementation
- All other 69 GAL methods are fully tested

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/gal-regression/native/gal_test_accessor.h

@@ -15,6 +15,7 @@
 namespace KIGFX {
     class OPENGL_GAL;
     class OPENGL_COMPOSITOR;
+    class SHADER;
 }
 
 // Test accessor functions - implemented in gal_test_accessor.cpp
@@ -26,4 +27,13 @@ unsigned int GetMainBufferHandle(KIGFX::OPENGL_GAL* gal);
 // Read pixels directly from the compositor's FBO
 bool ReadCompositorFBOPixels(KIGFX::OPENGL_GAL* gal, std::vector<uint8_t>& pixels, int* width, int* height);
 
+// Get the shader pointer for deactivating before fixed-function rendering (DrawBitmap)
+KIGFX::SHADER* GetGALShader(KIGFX::OPENGL_GAL* gal);
+
+// Deactivate shader to allow fixed-function pipeline (glBegin/glEnd) rendering
+void DeactivateGALShader(KIGFX::OPENGL_GAL* gal);
+
+// Reactivate shader after fixed-function rendering
+void ActivateGALShader(KIGFX::OPENGL_GAL* gal);
+
 #endif // GAL_TEST_ACCESSOR_H