x11: Don't share cached GLX visuals with GTK3

x11: Don't share cached GLX visuals with GTK3

We don't want to bind ourselves to GTK3 - both because we don't want to
accidentally cause bugs in a different codebase and because we want to
deviate from it.

While doing so, also store visuals as visuals and not as integers.
And only store one Visual because GTK4 only uses one visual.

And then remove the code that is leftover for dealing with the
compatibility Visual for GTK3.

PS: I'm kinda proud of my STRINGIFY_WITHOUT_BRACKETS hack.