egl_dri2: Match X11 visuals using rgba masks instead of depth

egl_dri2: Match X11 visuals using rgba masks instead of depth

Matching on visual depth to buffer size makes 8 bpc RGBA look similar to
10 bit RGB with 2 bit alphs - both have buffer size 32.  Instead, build
the rgba masks from the visual data and use that for finding matching
DRI configs.

We need to keep the special case that allows us to match 24 bit visuals
to DRI configs with buffer size 32.  We do that by creating an alpha
mask of "all the non-rgb bits" for 24 bit visuals and matching a second
time with that.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>