treewide: switch to glad2

Based on !130 (merged).

For most files (except eglinfo, wglcontext, wglgears, wglinfo and probably some that I forgot), all I did was running a regex:

gladLoadGL();((?:.|\n)*)(return 0); -> gladLoaderLoadGL();$1\n gladLoaderUnloadGL();\n $2;

(I am not expecting to merge this anytime soon... Probably we can merge it after a new release is made.)