audiodecoder: add _finish_subframe() method
This allows us to output audio samples without discarding any input frames, which is useful for some formats/codecs (e.g. the MonkeysAudio decoder implementation in ffmpeg which will might return e.g. 16 output buffers for an input buffer for certain files).
In the past decoder implementations just concatenated the returned audio buffers until a full frame had been decoded, but that's no longer possible to do efficiently when the decoder returns audio samples in non-interleaved layout.
Allowing subframes to be output before the entire input frame is decoded can also be useful to decrease startup latency/delay.
We don't strictly have to add new API for this, but I think it's clearer to do so.