Switch from libepoxy to glad2

Big ol' refactor of the entire OpenGL subsystem. Completely kills our dependency on libepoxy, in favor of a (bundled-via-submodule) generator based on glad2.

It's not the prettiest thing in the world, but this (finally) allows us to expose pl_opengl_params.get_proc_addr, and also avoids a heap of other headaches (apitrace, static libepoxy, thread safety, ...).