Fix Transorm Worker to shrink device bounds when doing scale-down

When doing a scale-down a great portion of the image becomes filled
with the default pixel. If the default pixel of a paint device is not
fully transparent, then the exactBounds() will not shrink automatically.
That is why we need to purge() all the default tiles of the paint device
here.

Note: the Crop visitor needs *not* the same stuff, since
KisPaintDevice::crop() calls KisDataManager::setExtent() which
explicitly drops all the tiles outside of the requested area.

Special thanks to Boud, who has found the real cause of the bug!