Check opus_decoder!=nullptr instead.

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

Reduce header dependencies.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Parse the R128_TRACK_GAIN comment string.

Call decoder_timestamp().  This is not necessary currently, but will
be as soon as we implement seeking.

This requires seeking to the end-of-stream, checking its granulepos,
and then seeking back to the previous file position.  We do this only
for local files.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Reduce heap allocator overhead.