Initialize the functions pointers in glad.c
When using glad as a static library on OSX, the linker will ignore the uninitialized variables during link, causing linking errors. See this [stackoverflow](https://stackoverflow.com/questions/19398742/os-x-linker-unable-to-find-symbols-from-a-c-file-which-only-contains-variables) for background on the issue. Can we add `= NULL;` to every declaration of a function pointer to solve this?