Fix bug 420338 by extending the selection extent.

This might be very buggy, but this in essence fixes https://bugs.kde.org/show_bug.cgi?id=420338

The actual bug is that QVector<QPolygon> KisOutlineGenerator::outlineImpl cannot generate an outline when the whole device sent to it is black. The whole device sent to it is black because the invert function does not extend the paint device to have some empty area to have 'selected', nor does it discard the select when it turns out invert just results in a blank. The latter is probably a better fix, but I cannot figure out how to do that. In fact, all boolean operations need to have this kind of check, if you select an area, and then perform a subtract-selection-operation over that whole selection, the same bug occurs (which this doesn't fix?)

/cc @rempt