Fix: texture alpha masking not functional after recent commits. Mask is
used to see if area has been covered by brush, so exclude the texture
alpha and texture alpha masks from mask comparisons. Coming next:
texture masks for 2d painting.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56421 954f8c5b-7b00-dc11-b283-0030488c597c