alsa-lib doesn't set errno, it returns errors as negative integers.

This check was added 9 years ago in commit
4dc25d39 to work around a dmix bug
which I assume has been fixed long ago.

Removing this fixes another corner case: if draining is requested
before the start threshold is reached, the PCM is still in
SND_PCM_STATE_PREPARED but not yet SND_PCM_STATE_RUNNING, which means
the submitted data will never be played.  This corner case is
realistic when playing songs shorter than the ALSA buffer (if the
buffer is very large).

This fixes a corner case which has probably never occurred and
probably never will: if Cancel() is called, and then Play() followed
by Drain(), the plugin should really play that data.  However
currently, this never happens, because snd_pcm_prepare() is never
called.

Closes #410

I added this sentence in commit
5271e81e, but this was merely
documented the legacy status quo, which has always been undocumented
for old-style filters.

But for new filters, using "==" for sub strings was a surprising
"feature", which I removed in commit
ac0852b4.

This fixes the assertion failure due to calling
AudioOutputSource::Close() twice.

Fixes a crash bug with `always_on` outputs which occurs because the
`AudioOutputSource` still has a pointer to an outdated `MusicChunk`.

Fixes #415

With the new command, the decision to pause or close the output moves
into the output thread.

This feature requires `libpcre`.

This call was missing, causing very high CPU usage when the ALSA
output plugin was used with dmix.

Closes #391

When `metadata_sent` is `false`, the plugin assumes there is metadata
which must be sent, even if no metadata page was passed to the plugin.
Initializing it to `true` avoids dereferencing this `nullptr`.

Fixes #412

Fixes potential compiler error when zlib is installed in a
non-standard directory.

Fixes #414

If the output is already open, the `current_chunk` pointer may be
bogus and out of sync with `SharedPipeConsumer::chunk`, leading to an
assertion failure in `SharedPipeConsumer::Consume()`.

Fixes #411

This fixes a valgrind warning because `buffer_before_play`
initialization needs to know the audio format from the decoder.

Meson always enables large file support on the compiler command line,
thus config.h doesn't need to be included anymore.  We'll remove the
whole `check.h` header soon.

Closes #409

`AVCodecParameters` contains values from the codec detected by
avformat_find_stream_info(), but after avcodec_open2(), a different
codec might be selected with a different `AVSampleFormat`.  This leads
to misinterpretation of data returned from FFmpeg, leading to random
noise or silence.

This was observed with FFmpeg 4.0.2 and a TS container file containing
MP2.  A mp3-float codec was detected returning `AV_SAMPLE_FMT_FLTP`,
but finally the `mpegaudiodec_fixed.c` was used, returning
`AV_SAMPLE_FMT_S16`.

By using the audio format from `AVCodecContext`, we ensure that MPD
and FFmpeg always agree on the actual audio format in the buffer.

This removes the FFmpeg bug workaround from commit e1b032cb which I
assume is obsolete after 7 years.

Fixes #380