Always set the FAST_PATH_NO_ALPHA_MAP flag for non-BITS images

Always set the FAST_PATH_NO_ALPHA_MAP flag for non-BITS images

We only support alpha maps for BITS images, so it's always to ignore
the alpha map for non-BITS image. This makes it possible get rid of
the check for SOLID images since it will now be subsumed by the check
for FAST_PATH_NO_ALPHA_MAP.

Opaque masks are reduced to NULL images in pixman.c, and those can
also safely be treated as not having an alpha map, so set the
FAST_PATH_NO_ALPHA_MAP bit for those as well.