Regenerate GL loader code with GLAD 2

GLAD 1 creates unusable loaders for EGL, while the newly released GLAD 2
does not, so for consistency I thought that it would be a good idea to
uniform things beforehand. While it had some API changes some renames
were all that was needed and everything works like before, at least on
the Wayland branch.

I've kept the structure identical, although this new generator has quite
a few hefty features, such as a single header mode.

I've also added GLAD to `thirdparty/README.md`, but I haven't specified
that in the commit title because it's a very small "fix".

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