feat(webgl): Add GAL native test harness for visual regression testing

Add a standalone test application that compiles KiCad's actual OPENGL_GAL
against system wxWidgets to generate baseline screenshots for comparing
native OpenGL rendering against WebGL rendering in the browser.

Architecture:
- Compiles 18 KiCad GAL source files from the kicad submodule
- Uses template-based private member accessor (safer than #define private public)
- Generates shader C++ files from GLSL (10 shader pairs for SMAA AA)
- Minimal stubs for KiCad dependencies (PGM_BASE, ADVANCED_CFG, etc.)

Test coverage:
- 11 scenarios testing ~19% of GAL API
- basic-lines, line-widths, circles, arcs, rectangles, polygons
- alpha-blending, transforms, grid-cursor, segments, complex-scene

Key insights documented:
- GAL uses world-to-screen transformation requiring 1:1 pixel mapping
- FBO reading required for clean screenshots on macOS
- Layer depth needed for proper z-ordering in complex scenes

🤖 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

@@ -0,0 +1,29 @@
+/**
+ * GAL Test Accessor
+ *
+ * Provides access to OPENGL_GAL internal members for testing purposes.
+ * This uses a technique to access private members without modifying KiCad headers.
+ */
+
+#ifndef GAL_TEST_ACCESSOR_H
+#define GAL_TEST_ACCESSOR_H
+
+#include <GL/glew.h>
+#include <vector>
+
+// Forward declarations
+namespace KIGFX {
+    class OPENGL_GAL;
+    class OPENGL_COMPOSITOR;
+}
+
+// Test accessor functions - implemented in gal_test_accessor.cpp
+GLuint GetCompositorMainBufferTexture(KIGFX::OPENGL_GAL* gal);
+void GetCompositorBufferSize(KIGFX::OPENGL_GAL* gal, int* width, int* height);
+GLuint GetCompositorMainFBO(KIGFX::OPENGL_GAL* gal);
+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);
+
+#endif // GAL_TEST_ACCESSOR_H