libavfilter/vf_yadif: Make frame management logic and options shareable

libavfilter/vf_yadif: Make frame management logic and options shareable

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.