glx: Remove Visual cache

glx: Remove Visual cache

1. We're using EGL most of the time anyway, so if we wanted to cache
   things, we'd need to port it there.
2. Our GL handling is massively configurable, so determining when to use
   the cache and when not is a challenge.
3. It makes startup nondeterministic and depend on whether a GTK4 app
   has previously been started on this display and nobody thinks about
   that when debugging.
4. The only benefit of the caching is delaying GL initialization - which
   made sense in GTK3 where almost no app used GL but doesn't make sense
   in GTK4 where almost every app uses GL.

So unless I find a big benefit to reintroducing it, this cache will be
gone for good.