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.

release/scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -1628,7 +1628,8 @@ class VIEW3D_PT_tools_projectpaint(View3DPaintPanel, Panel):
         stencil_text = mesh.uv_texture_stencil.name if mesh.uv_texture_stencil else ""
         row.menu("VIEW3D_MT_tools_projectpaint_stencil", text=stencil_text, translate=False)
         row.prop(ipaint, "invert_stencil", text="", icon='IMAGE_ALPHA')
-        col.template_ID(ipaint, "stencil_image", new="image.new")
+        col.template_ID(ipaint, "stencil_image")
+        col.operator("image.new").texstencil = True;
         col.prop(ipaint, "stencil_color")
 
         layout.prop(ipaint, "seam_bleed")