Default pixel and paint device bounds

[Marc]

Yesterday I fixed bug 245778 in the transform workers, though the
problem might still appear in other places.
Actually, the problem came from the fact that the first layer had a non
transparent default pixel. In that case, KisPaintDevice's exactBounds
and extent return defaultBounds (which is set to (0,0),(image width,
image height)) : that isn't really what the transform workers expect
when calling exactBounds.
For now, the transform workers check whether the default pixel is
transparent or not. If it isn't, then it uses the dataManager extent
instead of paintDevice bounds.
It might be good to consider changing the exactBounds() behaviour when
default pixel is not transparent (sven wondered whether we could unite
the default bounds with the data manager in that case).
Marc