Code cleanup:
Separate 2D painting functions away from brush.c, which should implement
the brush API and into its own file, paint_2D.c.
Ideally projective texturing and 2D painting should be totally separate,
however functionality is still interleaved mostly in the
BKE_brush_painter_paint function. The functionality of this function can
possibly be done using the brush stroke functions instead.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53492 954f8c5b-7b00-dc11-b283-0030488c597c