Allow setting method for GetProcAddress

[fsobolev]

While creating Avalonia example, SetupGL was causing problems, but copy-pasting the code only changing it to use Avalonia's gl.GetProcAddress made things work. I will add GetProcAddressFn to RenderContext to allow setting the method before calling SetupGL.

EDIT: Huh, don't know what I was doing wrong before, but I made it work in Avalonia without setting GetProcAddressFn. Anyway, can be useful for some edge cases I think (GLXGetProcAddress?). Actually it's needed on Windows