shaders: migrate GLSL to profile-neutral dispatch

[tritao]

Summary

Move Coin's GLSL implementation onto profile-neutral OpenGL dispatch.

• Add standard GLSL entry points to the shared GL glue with compatibility fallbacks where required.
• Migrate shader objects, shader parameters, and shader programs to that dispatch.
• Detect vertex and fragment GLSL support across core and compatibility profiles.
• Preserve the existing EXT-specific geometry-shader path.
• Validate compile, link, program use, and uniform updates on a core-profile EGL context.

Shader diagnostics and source attribution are intentionally kept in the following layer.

Stack

Layer 3/18, based on PR #12.