general: Support component alpha for all image types

Currently, if you attempt to use component alpha on source images or
images without RGB channels, Pixman will silently just use unified
alpha instead. This patch makes such images supported for component
alpha.

There is no particularly compelling usecase at the moment, but this
patch does get rid of a bit of special-case code both in
pixman-general.c and in test/composite.c.
Merge branch 'master' of git://anongit.freedesktop.org/git/pixman