Redrawing the picture on window expose

[Adrian Page]

Boudewijn Rempt wrote:
> If we have that, we can get rid of the projection is kis_image, which is as 
> big or bigger as the biggest layer, and which, I feel, is responsible for
> quite a few of our display artefacts. Just maintain the QPixmap, which 
> consists of the composed layers, and only compose the regions actually 
> damaged by Krita activity.
> 
> Are you working on this? Otherwise I might take a stab at it.

I've got the basic mechanism working, I just need to fix all the cases
where the pixmap needs updating. I should be able to finish it over the
weekend.

Adrian