Update GLAD to version 2

[totalgee]

This is just a nice-to-have, mainly to get rid of the annoying warnings that often show up under Windows (due to redefinition of APIENTRY when you need to include windows.h in a file, but Cinder has already indirectly included glad.h). The GLAD v1 explicitly defines APIENTRY in its header). In GLAD 2, they now use a different define within their library (GLAD_API_PTR), so this warning is gone.

The new GLAD (v2) generator is here: https://gen.glad.sh//
However, I suspect it will be a bit more complicated than just swapping in new generated headers and source files, because the GLAD API changed a bit (also it has more options, so I'm not sure which are best for Cinder).

Just adding this issue as a reminder.

[totalgee]

Just adding the link here to some of the comments in #2332 (which is not really necessary and I'm going to close now, since #2339 resolved the GLAD v1 compile warnings for now)... The bigger move to GLAD v2 will be left (in this issue) for a later time...