change the way caching is done in words a bit; instead of having one big QImage per page, split the image up in tiles. This makes redrawing things that are already cached a lot faster. I also got rid of the keeping around and re-using QImages in the cache, as that bit gets a bit more complicated with the tiling aproach (as not all tiles have the same size), and it didn't seem to have a very significant effect on speed anyway.