Implement KisRegion constructor for overlapping rects

Now we have KisRegion::fromOverlappingRects() which creates a region,
covering all the provided rects. The resulting region may be a bit
larger than the source rects by the providied tolerance. Such relaxed
requirements allow KisRegion be really much (2142 times!) faster than
QRegion.

We can/should use it for ouptimizing our updates sequences. We do not
care if updated area will be a bit larger, but we do care if overlapping
will be too high.