The bwdif filter can use common yadif frame management if we track
when a field is the first or last field in a sequence. While this
information is not used by yadif, the added benefit of removing the
duplicated frame management logic makes it worth tracking this state
in the common code.
I'm writing a cuda implementation of yadif, and while this
obviously has a very different implementation of the actual
filtering, all the frame management is unchanged. To avoid
duplicating that logic, let's make it shareable.
From the perspective of the existing filter, the only real change
is introducing a function pointer for the filter() function so it
can be specified for the specific filter.
