Overlay refactor:

Allow separate control for tex/mask/cursor overlay. This commit implements
separate overlays for mask textures and cursor curves. The user can turn on
and off separate parts of the overlay by using the appropriate widgets.
The cursor overlay widgets are located at the tool selection panel

Also fixed alpha masks not getting correctly masked and mask texture mapping
not having the correct update callback

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56222 954f8c5b-7b00-dc11-b283-0030488c597c