Multi-threading
I understand that on some platforms the function pointers are context-specific, however it should be possible to initialize one context per thread and then run independent renderers in each thread.

If I understand correctly, glad doesn't support this use case. However, I think it would be easy to support if the global function pointers, as well as any other static object in glad, is converted to use `thread_local` (C++11). This should be a straight-forward change if there is a macro that glad defines, which can then be used to inject `thread_local` in each definition.

Would that be possible?