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.
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