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>
