Fix #34970: in glsl mode the vertex paint face mask is not visible

Added face selection code to GLSL draw. We do this for Cycles
already, shall behave expected now.

Checked by Ton, thanks!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56131 954f8c5b-7b00-dc11-b283-0030488c597c