change how we store Region data internally

We used to keep the bounds of the region as a
separate rectangle. Instead we now store it as the last
element of the Vector<> of Rects.

This has the benefit of being slightly more efficient when
copying regions and reduces the overhead of small regions,
but more importantly will allow us to export the underlaying
SharedBuffer (eventually).

Change-Id: I80790e4fb1a09a747a5616000cfef852ac4ce9e9