Blender Git Loki

Fix T41141, can't paint texture in cycles.

The issue is actually that creating a new image in texture paint mode
will set it always as a stencil image. Internally, the code checks if
the painted image is the same as the stencil and if it is, no painting
is done.

Solution is to expose a boolena to the operator for setting the image as
a stencil (could be an enum in th future for more uses)

Stencil UI is a bit weird here, will definitely redesign.