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Audio output plugins can use this to expedite discarding pending
buffers. This reduces latency upon seeking or stopping the input.

This new callback also supports explicit draining, but this is not
used for the time being.

The date parameter is always zero, remove it.

With only one input per output, the lock is only useful:
 - to restart the audio instance (needed 3 out of 4 locks anyway),
 - to propagate volume change to the mixer or output plugin,
 - to access the FIFO from the output plugin thread (if applicable).

So 4 fine-grained was over-engineering. Most importantly, the locking
scheme was overly complicated and generally misunderstood/misused. Also
to avoid lock inversion, some unlocking/relocking sequences were
introduced; they broke atomicity.

We could certainly reduce the scope of the remaining lock. Since we
have one only input per output, most of the code is only ever run from
the decoder thread. Thus reentrancy is not anymore needed in some
places. But first aout_Restart() needs to be fixed and simplified.

This is required for PulseAudio synchronization and to pause/resume
with low-latency. This should also be useful for other buffered
 audio outputs such as ALSA or CoreAudio.

This has no effect other than wasting CPU and memory. Audio filters are
supposed to reallocate their buffers as needed.

This improves mute flag handling in the PulseAudio output:
We do not blindly reset the mute flag.

The input resource creates the aout instance with the input manager as
its parent object.

If this fail, the aout input should not be created at all. This removes
the fallback to an aout instance with the decoder object as parent.
This would potentially crash as the decoder is shorter-lived than the
aout instance.

 It closes #4278, partially closes #3152, closes last part of #3090.
 It does not fix audio losses due to potentially buffered data by
packetizers/decoders.

No functionnal changes.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

audio filter plugins are now useless.
This breaks visual (needs to be ported to filter_t) and will likely
expose bugs in under-used audio filter2 infrastructure.

TODO: use filter chain instead of custom filter handling in core.