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.

Not necessary.

The previous commit was broken.  D'oh!

The IsActive() method returned true even if the timer was not active,
after it completed once.  This broke the state file timer, and the
state file was not saved periodically.

Check for STOP before decoding the first chunk.  This reduces the
command latency.

Remember the chunk's total size.

Nobody cares.

Eliminate the loop from dsdiff_stream_decode().  It makes the code
complex, real-world files with multiple DSD chunks are outside of the
specification, and the "chunk_size" variable would be bogus anyway.