one last bit of iterator weirdness

[Boudewijn Rempt]

Apart from the blur filters not giving exactly the same result as before 
-- which breaks the KisConvolutionFilterTest and KisProcessingsTest -- 
there is one last bit of weirdness.

In KisPainter::bitBltImpl, we used to create a CONST iterator on the
srcDev:

KisRandomConstAccessorPixel srcIt = srcDev->createRandomConstAccessor(srcX, srcY);

and then we would, depending on template parameter take either the rawData 
or the oldRawData of that const iterator. NG const iterators only have 
oldRawData.

If I change the iterator to non-const, suddenly undo is broken on other 
layers than the first!

I'm really scratching my head here -- I cannot explain what is happening.

Boudewijn