For type safety and code readability.

After seeking, the MPD core automatically refreshes the timestamp, and
thus discards the value from decoder_timestamp().

Avoid roundtrips to floating point.

Move to fixed-point integers instead of floating point.

This check was redundant, because we could only exit the loop when
ret==DECODE_OK.

This flag is never read.

Translate SampleFormat::DSD to SND_PCM_FORMAT_DSD_U8, which was added
to alsa-lib 1.0.27.1.