X8L8V8U8 support should be common. Some more recent cards
do support this format, but not L6V5U5.
Add fallback for this format to have it alwaus supported.
L6V5U5 conversion rule apparently differs a bit from the normal
spec, and thus the gallium equivalent format leads to slightly
wrong colors. Since some recent cards do not support it, do not
support it either.
Signed-off-by: Axel Davy <axel.davy@ens.fr>
Axel Davy
10e548c0c9