The API for `pl_frame_mix` is currently a bit abstract and high-level.
Most clients will probably not care to reimplement the massive amounts
of logic that go into constructing a proper frame queue.

Enter `pl_queue`, the solution for all your frame queuing needs! This
will interface with the decoder via your choice of a push or pull API,
and translate all decoded frames into an abstract internal timeline,
`pl_frame_mix` slices of which can be probed for arbitrary timestamps.

As an extra bonus, this also contains code for things like FPS
estimation and VPS (vsyncs per second) estimation, somewhat mirroring
the logic in e.g. mpv's interpolation/display-sync code.

It's worth pointing out that this is a rather severe API break for
`pl_frame_mix`, but I'm pretty sure this API has absolutely zero users,
given the massive amount of effort required to use it before this
commit. So I didn't bother with backwards compatibility, aside from an
API bump as usualy.