refactor(webgl): Move WebGL GAL from test harness to KiCad source tree

Migrate WebGL GAL implementation from tests/gal-regression/wasm/webgl/
to kicad/common/gal/webgl/ and kicad/include/gal/webgl/.

This integrates the WebGL GAL properly into KiCad's build system:
- Update test Makefile to use sources from kicad/ instead of local copies
- Update build scripts for new source locations
- Add test-gal-webgl.sh script for running WebGL regression tests
- Update Docker to Emscripten 4.0.22

The WebGL GAL passes all 28 regression tests (matching baseline).

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

docker/Dockerfile

@@ -1,5 +1,5 @@
 # Use ARM64-native image for Apple Silicon (M1/M2/M3/M4)
-FROM emscripten/emsdk:4.0.2-arm64
+FROM emscripten/emsdk:4.0.22-arm64
 
 # Install build tools required for KiCad WASM build
 RUN apt-get update && apt-get install -y \