Test for rectangular artifacts on transparency masks
The problem itself is not solved yet. Only a test that
can reproduce it in a stable way is implemented.
The only things that I know right now is:
- The problem is related to multithreading. It doesn't appear when the number of threads is limited to 1.
- Artifacts appear only after clearing the transparency paint device.
- Artifacts are neither present in the transparency mask paint device, nor in the projection of its parent layer.
- Artifacts appear in the final image projection, right when the async merger calls layer->projectionPlane()-> ->apply().
- Changing dirtyRect in KisToolUtils::clearImage() to the entire image bounds doesn't help much.
CCBUG:395273