Fix part of #34640: vertex color blur brush would average face corner colors
at vertices for the entire mesh, which meant that discontinuous face colors
would be lost for unrelated parts of the mesh too. Now only vertices within
the brush radius will be affected.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56295 954f8c5b-7b00-dc11-b283-0030488c597c