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/scenarios/gal_test_scenarios.h

@@ -0,0 +1,40 @@
+/**
+ * GAL Test Scenarios
+ *
+ * Defines test scenarios for visual regression testing of GAL rendering.
+ * These scenarios use KiCad's GAL API to test the rendering pipeline.
+ */
+
+#ifndef GAL_TEST_SCENARIOS_H
+#define GAL_TEST_SCENARIOS_H
+
+// Forward declaration
+namespace KIGFX {
+    class GAL;
+}
+
+namespace GALTest {
+
+/**
+ * Get the number of test scenarios
+ */
+int GetScenarioCount();
+
+/**
+ * Get the name of a scenario
+ */
+const char* GetScenarioName(int index);
+
+/**
+ * Render a scenario using KiCad's GAL API
+ *
+ * @param gal Pointer to the GAL instance (OPENGL_GAL or WEBGL_GAL)
+ * @param index Scenario index
+ * @param width Canvas width in logical pixels
+ * @param height Canvas height in logical pixels
+ */
+void RenderScenario(KIGFX::GAL* gal, int index, int width, int height);
+
+}  // namespace GALTest
+
+#endif // GAL_TEST_SCENARIOS_H