If an application switches between render targets of a different size, but
with the same depth/stencil surface it'll typically clear the depth/stencil
surface before drawing. However, in case of the smaller render target that
wouldn't be a full clear, so we'd have to do a depth copy if we also switched
between onscreen and offscreen rendering. Keeping track of which part of the
depth/stencil surface is current for onscreen/offscreen allows us to avoid
most of these kinds of copies. The current scheme requires the current/dirty
rectangle to have an origin at (0,0). This could be extended to an arbitrary
rectangle, but the bookkeeping becomes somewhat more complex in that case, and
it's not clear that there would be much of a benefit at this point.