remove GL / GLES dependency from projectMConfig for any platform other than WIN32

[mbaetgen-wup]

No description provided.

[kblaschke]

Does Windows really need to hard-link against opengl32.dll anymore? AFAIK the glad loader already loads it via LoadLibrary(). Apps using projectM will have to link or resolve GL calls anyways, so it's not really necessary to propagate the CMake dependency IMO.

[mbaetgen-wup]

as far as I know it's needed, but I will test it to make sure

[kblaschke]

as far as I know it's needed, but I will test it to make sure

The test UI executable may need the dependency explicitly (but GLEW and the associated init call can be removed if not already done so).

Otherwise, glad loads opengl32.dll via LoadLibrary(), thus a hard linker dependency shouldn't be required here. Apps may do the same so we shouldn't force them to hard-link the library if not absolutely necessary.

[kblaschke]

Tested it, the SDL2 test app doesn't need OpenGL dependencies either as SDL2 also loads OpenGL dynamically as well. So fully removing the OpenGL dependency from the build system is fine, as it's rather a runtime-only dependency now. This will certainly include Windows as well.