Fix sculpt getting slower as you paint a longer stroke. Partial redraw was
redrawing the whole area that was painted on from the start of the stroke,
should only do the last part.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56767 954f8c5b-7b00-dc11-b283-0030488c597c