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/generated/glsl_kicad_frag.h

@@ -0,0 +1,13 @@
+// Auto-generated shader header from kicad_frag.glsl
+#ifndef GLSL_KICAD_FRAG_H
+#define GLSL_KICAD_FRAG_H
+
+#include <string>
+
+namespace KIGFX {
+namespace BUILTIN_SHADERS {
+    extern std::string glsl_kicad_frag;
+}
+}
+
+#endif // GLSL_KICAD_FRAG_H