Define NOMINMAX before windows.h

[AlexMax]

I ran into an issue where GLAD included windows.h. I've read #42, and I understand why you thought it was a good idea to keep it, but one thing that would help a whole bunch is defining NOMINMAX before including windows.h. This specifically is a major pain-point when including windows.h, as the min and max macros conflict with std::min and std::max, which seems like behavior you would want to avoid in most cases. Those who needed Microsoft's min/max could simply include windows.h before GLAD.

For what it's worth, SDL_opengl.h appears to do exactly what I describe.

[Dav1dde]

I will look into it tonight, seems like I missed that change as it was proposed in #42. Thanks for bringing that up again.