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.

They're mostly the same, except for the GL version also destroying its
contexts, and being a bit more careful about the order in which backbuffers
are destroyed.

    
Lying about the number of contexts a swapchain has isn't very useful.

It adds COM overhead and validation of the swapchain index, but we already
know the index is valid.