Replace GLEW with GLAD

[Xottab-DUTY]

Benefits:

• OpenGL 1.0 to 4.6
• OpenGL ES 1.0 to 3.2
  • Possibility to implement OpenGL ES support: https://github.com/OpenXRay/xray-16/issues/280#issuecomment-2136446760
• Handles function aliases
• One header file
• Carries no dependency (except for user provided function to load OpenGL extension. We use SDL_GL_GetProcAddress)
  • GLAD sources are integrated into our build system. This may be considered as disadvantage, but it gives us more portability.
  • Removes dependency from GLEW, which may be missing on some platforms (e.g. latest 32-bit Ubuntu versions)

The generated glad.h includes all available extensions, including GL_ARB_separate_shader_objects, but excluding GL_EXT_separate_shader_objects due to a conflict. I decided that it would be best to not include it at this moment (if ever). Read more here: https://community.khronos.org/t/confused-gl-active-program-gl-current-program/65477.

[Xottab-DUTY]

@sobkas, @AMS21, please look at it! Does it work under Linux?

[AMS21]

@sobkas, @AMS21, please look at it! Does it work under Linux?

Compiled with gcc 14.1.0 and seems to work fine on my machine :)