1) Implement KisPaintDevice::sequenceNumber() which basically lets
   us know when the device has changed

2) Implement caching of the filtered paint device in KisColorizeMask.
   Now we can check for the sequence number and reuse the cache in
   case it didn't change.

3) Implement automatic selection of the mask composite op depending on
   the number of transparent pixels in the parent layer.

Ref T372