Slight cleanup for 2d painting. Cache is always enabled (used one of the
bookkeeping variables from when the code was shared with texture
painting) Also texonly now is always zero, probably another leftover
from 2d/3d painting entanglement era.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55669 954f8c5b-7b00-dc11-b283-0030488c597c