Patch for smudge

[LukasT.dev@gmail.com]

> Here is a new version of the patch that fix one regression (that when you
> smudge with changing size, you might get "old" colors back, I don't
> remember the reference of the bug :) ).

Can you please commit it? It is currently the closest solution to our majer 
bugs with smudge.

The blocky effect is fixed in copy2 now.
 
> However, the apparition of black around the edge of smudge is worrying. I
> am starting to wonder if we shouldn't use a different composite op for
> copyPainter. An inbetween COPY and NORMAL.

You mentioned on IRC that we do not respect the opacity of the pixel. Just for 
the record. Also there isn't support for channels flags yet in composite copy2 
yet.