Fixes regression by commit 4e2a551f

Closes https://github.com/MusicPlayerDaemon/MPD/issues/599

Closes https://github.com/MusicPlayerDaemon/MPD/issues/604

Closes https://github.com/MusicPlayerDaemon/MPD/issues/607

This exception is usually thrown by class DecoderBridge, but the Opus
plugin (ab)uses it as well, so we need to catch it.

Just in case.

Prepare for a new mode which may convert to a different sample format
when applying volume, to reduce dithering.

It's not a tri-state anymore since we introduced C++ exceptions.

Fixes regression by commit 28e07e90

Closes https://github.com/MusicPlayerDaemon/MPD/issues/596

Pass only the amount of data to PcmExport::Export() when its full
output fits into the ring buffer.  Using only a part of the
PcmExport::Export() result may cause data corruption because
PcmExport's internal state may contain partial blocks which would need
to be rolled back when only some of its output data was used.

As a side effect, this fixes an assertion failure because
PcmExport::CalcInputSize() considered partial block data and could
cause Play() to return a number larger than the "size" parameter.

If snd_pcm_writei() fails, throw an error and stop playback instead
of going into an endless busy loop.