new weight paint draw option to display unweighted vertices with the option to check on the active group or all groups.

notes:
- vertices with zero weights are considered the same as vertices outside of a group.
- currently these show black but this can be made a theme color.
- multi-paint overrides this option (noted in description)


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54741 954f8c5b-7b00-dc11-b283-0030488c597c