Support for brush overlay in texture paint mode. For the image editor,
the tiled overlay is still a bit off, investigating on how to better
correct this, though it may well be non trivial.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55668 954f8c5b-7b00-dc11-b283-0030488c597c