Certain programs can start the virtualcam filter, then they may choose
to call `Stop()` on the filter, call `SetFormat()` to change the
resolution, then call `Run()` again to start the filter again. The
Windows virtual camera filter did not account for this, thus if the
resolution was different, it had potential to cause a crash.
To fix this, store the last filter resolution, then check the resolution
every frame, and if it changes, reset the scaling information.
(Author note: This code is unclean. What we need to do with the virtual
camera filter is make it only create the thread on `Run()`, then join
the thread on `Stop()`. It's currently a bit complicated to make it do
that at the moment, so this code is a kind of an annoying stopgap for
now.)
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