Merge lots of duplicate code.

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.

Eliminate the temporary buffer, do I/O right into the WritableBuffer.

Replaces GLib's g_ascii_strncasecmp().

Replaces GLib's g_ascii_strcasecmp().