This will allow us the get rid of the swapchain refcounting hacks in d3d9 in
particular. This is similar to the way we handle resources that are still in
use by a stateblock, but aren't referenced anywhere by the application.
This prevents for example a d3d9 depth stencil from being destroyed when it
has no external references but is still in use by the device/stateblock. A
nice side effect is that it simplifies handling of "implicit" surfaces like
the frontbuffer and backbuffers, as well as the forwarding of reference counts
for surfaces that are part of a texture.
Henri Verbeet