* app/clone.c: more cursor support.
* app/cursorutil.[ch], cursors/{bad,badmsk}: new cursor
* app/paint_core.c: fix that rounding error the right way this time.
* app/pixel_processor.c, app/pixel_region.c: Lock the tiles while
they are being processed. Only create new threads if the region
being processed is large enough to warrant it.
