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:

1) The problem is related to multithreading. It doesn't
   appear when the number of threads is limited to 1.

2) Artifacts appear only after clearing the transparency
   paint device.

3) Artifacts are neither present in the transparency mask
   paint device, nor in the projection of its parent layer.

4) Artifacts appear in the final image projection, right
   when the async merger calls layer->projectionPlane()->
   ->apply().

5) Changing dirtyRect in KisToolUtils::clearImage() to the
   entire image bounds doesn't help much.


CCBUG:395273