The generic is<T>() uses dynamic_cast which is fine in the majority
of cases, but when one of them shows up in profiles, we can make it
faster by answering the is-a question manually.
To support this, the GUI process and the WebContent service will now
coordinate their backing store bitmaps. Each backing store can be
referred to by a serial ID, and we don't need to keep resending it
as a file descriptor.
We should probably do something similar in WindowServer. :^)
