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 pristine Khronos core OpenGL and WGL headers privately, with exact upstream provenance.
• Keep Inventor/system/gl.h as Coin's public OpenGL abstraction and isolate compatibility enum fallbacks in gl-fallbacks.h.
• Select private core-profile headers only while compiling Coin; installed consumers continue to use normal platform OpenGL headers.
• Add explicit LegacyGL build configuration while keeping compatibility rendering enabled by default.
• Publish the installed COIN_HAVE_LEGACY_GL_RENDERER capability separately from the build option.
• Validate the installed CMake package through a standalone external consumer.
• Add initial LegacyGL-disabled core-header/glue coverage without claiming that the complete library is optional yet.
• 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 position

Bottom layer of this stack, based on freecad-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.

[tritao]

Thanks for the detailed review. I addressed the feedback as follows:

• Kept the Khronos headers private and byte-for-byte identical to the recorded upstream revision.
• Kept installed consumers on normal platform OpenGL headers.
• Simplified the profile-aware header selection.
• Left basic.h.in unchanged (we will remove it with that other Automake cleanup PR later).
• Moved COIN_BUILDING_COIN to gl: add profile-neutral GLSL dispatch and core test support #65 (never intendeded for that change to end up here in the first place)
• Switched CI to Ninja and parallel builds.
• Kept pull_request: {} as the explicit unfiltered trigger.
• Documented the standalone installed-package smoke test.
• Simplified comments and corrected the PR description and base.

[Lgt2x]

we will remove it with that other Automake cleanup PR later

is this coin3d#641 or something else? I believe a merge of upstream master into freecad-master is in order

[Lgt2x]

As specified by Volker, you can drop this copyright in new files (or those completely modified like this one) and assign yourself or "FreeCAD contributors". Can you do a sweeping change on the PR stack?

[Lgt2x]

what is this driver version? Looks old, nowadays we're in the 5XX range

[Lgt2x]

not sure why we use such an old commit. I was unable to find the exact commit hash where the files are imported from, 56a53de has a slightly different glcorearb.h. I think this is important for verification to easily match the imported headers' hashes against official headers.

[Lgt2x]

this is a 2017 commit, and the file at this commit is not the same as the one imported, which looks like a more recent version. It would be best to document the actual commit where this is from rather than the one presented as the 'semantic' one

[Lgt2x]

You can freely merge once the last comments are addressed, it's mostly details, only gl headers import traceability is important to me.