Draft: vout/xcb: map 8-bit RGB with mask to common RGB8 chromas
Draft on top of !4233 (merged) to have the 3 common RGB8 chromas.
v8: remove the fix the blue mask checks, they are not used in this MR, they should be fixed when handling 15/16 bits.
v9: Draft on top of !4269 (closed)
Edited by Steve Lhomme