Yet another bug. This time filters vs selections

[Boudewijn Rempt]

More notes: you cannot simply feed a filter a chunk of pixel bytes,
filters need to know the location of the pixel they are working on
because they often need pixels at a certain location relative to the
current location, so we will go on giving filters a KisPaintDevice
and a rect to handle, so they will need to go on using iterators.

But the good thing is, we can make iterators a lot faster by cleverer
caching.

Boudewijn