Now that we have an OpenGL compatibility profile we might want to use
OpenGL compatibility symbols that are not available in our current glad.
This commit has been generated with https://glad.dav1d.de/ with all
extensions enabled and OpenGL 4.6 compatibility profile.
The main advantage of switching to glad from glLoadGen is that, apart
from being actively maintained, it supports a customizable entrypoint
loader function, which makes it possible to also support OpenGL ES.