use GLAD to link OpenGL

[mbaetgen-wup]

Changes

• Add GLAD as vendor lib with static linking
• Replace projectM system GL dependencies with GLAD
• Remove GLEW
• Add C api function for creating projectM with an app supplied resolver for GL function pointers
• Add loader class to handle GL function pointer discovery
• Adjust SOIL2 GL discovery to integrate with projectM/GLAD
• Adjust sdl-test-ui to discover GL functions from SDL

Todo

• Re-generate GLAD with required versions / extensions (currently gl 3.3, gles 3.0, egl 1.4, wgl 1.0, no extensions, alias =true, loader=true)
• Continued testing and clean-up

Test status - OS/GL API

• GLX: ok
• GLES (Linux): ok
• WGL: todo
• EGL: todo
• Android: todo
• Emscripten: todo
• MacOS: todo

Test status - framework integrations

• Gstreamer: ok
• SDL2: ok
• SDL3: todo
• Qt: todo