produce correct color channel descriptor for 32 bit depth
vesa modes. the vesa bios omits reserved byte pos/mask
in the mode info for 32 bit, resulting in a r8g8b8 channel
descriptor that doesnt add up in length to 32 bit depth.
this patch will produce the correct x8r8g8b8 channel by
assigning the unmasked part (not used by any rgb component)
of a pixel word to the x channel.
it also doesnt assume any channel ordering and should work
for all rgb modes (16/24/32).