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).
02f3b9312a