Reevaluate glext.h dependancy

[bretjackson]

• Does glext.h need to be distributed with MinVR? Can we switch on/off this dependency based upon the need for GPU affinity or not? Whatever we implement for the checkerboard and interlaced stereo situations would likely also require GL extensions. Are these the only places where we need something more than super basic OpenGL? I wonder if these should/could be optional dependencies to really make MinVR Core as bare bones as possible.

I'm not sure about this. I ended up running into issues with windows because it doesn't support anything above opengl 1.1 out of the box without adding extensions. There is some nasty code I had to add to RenderThread to load a bunch of functions to get the shaders, framebuffers, and textures to work on windows without adding glew as a dependency. We should probably do some testing, and at the very least might be able to ifdef it out if you aren't on windows.