Port kpDocument::getSelectedPixmap() for non-rectangular selections.
This logic is moved into the new kpSelection::givenImageMaskedByShape(),
to group as much selection code together as possible.
Factor code out into kpSelection::maskRegion().

We probably have off-by-one errors with the selection border.

In user terms, this means that you can now select an area containing transparent pixels and then Image / More Effects ...

Without XRENDER, this means you can now select any area and actually get something reasonable in Image / More Effects, rather than a yellow blob.