3D mapping for projective texture painting (only for draw brush). Useful
to draw with procedural textures on surface of object. 2D painting will
still paint as if tiled.

When we unify the paint systems, the texture sampling functions will
need to be changed. Sculpt uses a slightly different system that passes
both screen and 3d coordinates to the sampling function. This commit
however is not too disrupting for that however so it can go in now.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53932 954f8c5b-7b00-dc11-b283-0030488c597c