Rename "shapeRegion" to "boundingRegion"

This is more accurate, and we want to introduce an input
shape region sometime soon.
Remove Region.from_rectangle

It's unused, and hasn't been as useful as I thought it would be.
Don't copyArea if the position doesn't change

It won't help in this case.
Use pixman's math for damaging windows for redraw
Remove pixman_region_subtract_region

It doesn't exist...
Put the bounding region in window space

If we want to have the bounding region modifyable by
clients, it makes sense to do this.
Clean up code

We never call calculateAbsoluteOffset without including ourselves,
so remove the extra parameter.
Implement support for basic window shapes