Calculate the size of sub-region that needs repainting, which can be a lot smaller than the region itself.

Greatly improves playing content of <a href="http://www.cwi.nl/projects/Ambulant/Demos/Euros.zip">http://www.cwi.nl/projects/Ambulant/Demos/Euros.zip</a>; and the fadeIn trans-in (other transitions simply don't work yet :)