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.
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