Fix missing brush texture mask in texture properties, for cycles. Handling this
properly with blender internal is probably for after release, when we can add
the same system to gather textures from brushes, modifiers, force fields, ...

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56442 954f8c5b-7b00-dc11-b283-0030488c597c