GdkGLContextX11: Ensure we get the fbconfig with the exact same visual

We really want a gl context with exactly the same visual, or we will
get a badmatch later, which hits us on nvidia as per:

https://bugzilla.gnome.org/show_bug.cgi?id=738670
Pick the first rgba visual, not the last

The visuals are typically sorted by some sort of "most useful first"
order. And picking the last one is likely to give us the weirdest
matching glx visual.
GdkGlContextWayland: Don't require specific bit depths

This just means we may fail to get a gl context, we just require
*some* color depth.