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.

gstreamer/gst-libav#49
8 jobs for audiodecoder-finish-subframe in 39 minutes and 17 seconds (queued for 74 minutes and 25 seconds)
Status Job ID Name Coverage
  Preparation
passed #154666
gst indent

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passed #154665
manifest

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  Build
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build fedora x86_64

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  Test
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check fedora

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  Full Builds
passed #154669
build android arm64 api28

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build cerbero cross win32

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passed #154672
build cerbero cross win64

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build cerbero fedora x86_64

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