Move the handling of the future/past frames and the associated dataflow
rules to a separate source file.
While this on its own seems rather questionable and just inflates the
code, I intend to reuse it for other filters. The logic is annoying
enough that it shouldn't be duplicated a bunch of times.
(I considered other ways of sharing this logic, such as an uber-
deinterlace filter, which would access the hardware deinterlacer via a
different API. Although that sounds like kind of the right approach,
this would have other problems, so let's not, at least for now.)
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