So that we have fewer cases to deal with when we transition to
32-bit bitmaps.
(*) rename maMask to maAlphaMask, since now it is only being used
for alpha duties.
(*) drop mbAlpha and mbTransparent to simplify state management,
the only thing we need to check for alpha is if maAlphaMask is
non-empty.
Change-Id: I06252e38e950e846a94b4c2ba8ea763be17801fe
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/111679
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
ddd2639a48