xcb/x11: clean up search for X11 visual
Pixmap formats are not indicative of what the screen can actually show. For instance, 1 bit formats are typically used for pixel masks, but can not be painted. Instead look for a supported depth, then a supported visual for that depth. Also check (in the previous commit helper) the RGB masks actually corresponds to a VLC pixel format. We would not want to treat R10G10B10A2 as R8B8G8A8. This changeset effectively drops RGBA support for the time being.
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