build: establish profile-aware OpenGL headers

[tritao]

Summary

Establish the profile-aware OpenGL header and build-configuration foundation used by the rest of the renderer stack.

• Vendor Coin-owned Khronos core OpenGL and WGL headers with recorded provenance.
• Add explicit LegacyGL build configuration while keeping compatibility rendering enabled by default.
• Separate the internal build decision from the installed `COIN_HAVE_LEGACY_GL_RENDERER` capability.
• Route core-only builds and installed consumers through Coin's namespaced Khronos headers.
• Extract portable OpenGL enum fallbacks from the legacy monolithic header.
• Add installed-package validation and initial Legacy-renderer-disabled build coverage.
• Allow CI and CodeQL to run on stacked pull requests.

This layer establishes header and configuration policy only. It does not yet make LegacyGL traversal or implementation units optional.

Stack

Layer 1/18, based on master.

[Lgt2x]

Was the target branch change intentional @tritao ? The diff is borked

[tritao]

Was the target branch change intentional @tritao ? The diff is borked

Not intentional, it got screwed up somehow, fixed now.

[Lgt2x]

it changed again @tritao :(

[tritao]

Fixed again.