* 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.