Add 24/16/15 fixed position RGB chroma and use them where possible.
This is a mix of !4119 (closed), !4225 (closed), !4183 (closed) and !4201 (closed) but keeping the RGB+mask handling where it's necessary.
RGB+mask with a default or set mask are replaced with their equivalent with fixed component position.
In the end a few code simplification is done to remove the code where all masks are now 0.
v7/v8: removed some 15/16 bits assumptions on endianness, they may be wrong or right, no idea.
Edited by Steve Lhomme