We're doing this all wrong.
We've asking for "at least 1 bit" of A, R, G, B color depth.
ARGB2101010 fits that nicely, so mesa on radeon gives it to us.
This only fixes the drop shadows though, it's entirely possible that
a fullscreen window without alpha would get ARGB2101010 instead of
XRGB8888, so this code probably needs a rethink for multiple engines.