X11: Pick better system and rgba visuals for GL

We want to create windows with the default visuals such that we then
have the right visual for GLX when we want to create the paint GL
context for the window.

For instance, (in bug 738670) the default rgba visual we picked for the
NVidia driver had an alpha size of 0 which gave us a BadMatch when later
trying to initialize a gl context on it with a alpha FBConfig.

Instead of just picking what the Xserver likes for the default, and just
picking the first rgba visual we now actually call into GLX to pick
an appropriate visual.
Cache default gdk visuals in the GDK_VISUALS property on the root window

This means we don't have to try to initialize opengl in every gtk
instance that is stated. It will only happen for the first one.

https://bugzilla.gnome.org/show_bug.cgi?id=738670
GdkX11GLContext: Fix leak