The clone tool in the image editor can show a second texture on top
of the image. This wasn't ported and now results into alpha and depth
issues. This fix adds the clone tool drawing to the overlay engine.
I checked 3 approaches to this issue:
- Do the stencil drawing in the image engine, but that leads to checking overlay flags and tool flags in the image engine but removes the need for another shader.
- Add a separate overlay module (or combine with overlay_image) but that lead to strange results in code style as the overlay_image is targetting the 3d viewport.
- Include the image rendering in the overlay_edit_uv as this issue is closely bound to the image/uv editor.
For this solution we chose to implement the option 3, but can easily be
transferred to the overlay_image as that fits what we had done for D8234.