There is no always need for new buffers.

Fairly straightforward. We just need to modify the scaler handling code
slightly to support looking at a different list of filter presets.

This exposes libplacebo's frame mixing functionality to vf_libplacebo,
by allowing users to specify a desired target fps to output at. Incoming
frames will be smoothly resampled (in a manner determined by the
`frame_mixer` option, to be added in the next commit).

To generate a consistently timed output stream, we directly use the
desired framerate as the timebase, and simply output frames in
sequential order (tracked by the number of frames output so far).

To present compatibility with the current behavior, we keep track of a
FIFO of exact frame timestamps that we want to output to the user. In
practice, this is essentially equivalent to the current filter_frame()
code, but this design allows us to scale to more complicated use cases
in the future - for example, insertion of intermediate frames
(deinterlacing, frame doubling, conversion to fixed fps, ...)