Uncomment //#define CACHE_EXACT_BOUNDS and KisPaintDevice will attempt to cache the exact bounds of a paintdevice through the setDirty calls.

This is not perfect: filled shapes stop working, for instance. I also do not notice any speed improvements, but I'll leave the switch in for Cyrille to play with when he wants to profile next.

And who knows -- we may get it working after all. It makes about 15 to 45 ms difference when selecting.