Add stencil control for mask overlay. Key combinations are the same as
regular stencil but use Alt as well, so Alt-Rclick is translation, Alt-
clamping to avoid scaling of stencil to zero.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56230 954f8c5b-7b00-dc11-b283-0030488c597c