Fix "can't paint" bug no.1, painting with black on image editor did not
paint. Was own regression when optimizing colour operations. I will not
use an alpha bit mask since it may run into portability issues with byte
order.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55358 954f8c5b-7b00-dc11-b283-0030488c597c