avauddec: fix decoding of APE audio
.. and other formats where ffmpeg gives us multiple subframes per input frame.
Since we now support non-interleaved audio, we can't just concat buffers any more. Also, audio metas won't be combined when buffers are merged, so when we push out the combined buffer we'll look at the meta describing only the first subframe and think it covers the whole frame leading to stutter/gaps in the output.
We could fix this by copying the output data into a new buffer when we merge buffers, but that's suboptimal, so let's add some API to GstAudioDecoder to push out subframes and use that instead.
Edited by Tim-Philipp Müller