This patch allows mpd to recognise the albumartist tag in the way
foobar2000 and others write it to files.

Splitted flac_copy_vorbis_comment() into flac_copy_comment() and
flac_copy_comment().

Simplify flac_copy_vorbis_comment() by moving the comment
identification code out.

Free the tag object when it turns out to be empty.  This simplifies
several functions and APIs.

Use tag_is_empty() instead.

Renamed types, functions, variables.

This patch allows mpd to recognise the albumartist tag in the way foobar2000
and others write it to files.

Don't depend on the daemon's locale settings.  Comment names are
ASCII.

vorbis_parse_comment() should be a function which converts one comment
to a tag item.  It should do everything required to do the conversion,
including looping over all possible tag types.

Eliminate some duplicate code.

Always allocate a new tag object before parsing the vorbis comments;
free it when it turns out to be empty.  This simplifies the code a
bit.

Renamed functions and variables.

Make ogg_parseCommentAddToTag() return bool instead of unsigned int.

"#ifdef G_BYTE_ORDER == G_BIG_ENDIAN" cannot work, of course.

Removed all allocation functions, xwrite(), xread(), ARRAY_SIZE().
Those have been superseded by GLib.

Use G_GNUC_UNUSED instead of mpd_unused (which has already been
removed).

Use GLib's G_GNUC_UNUSED instead of gcc.h's mpd_unused.

Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.

I tried to search for a certain composer in my collection, but only
non-mp4 files showed up. The source code reveals that this tag is not
read. This can be fixed by reading the 'Writer' tag field, in
mp4_plugin.c, in function mp4_load_tag.

I actually tried this, and after compiling with those lines added,
also mp4 (.m4a) files showed up when searching for a composer.

Don't do two allocations for the mod_Data structure.

This patch adds RVA2 (relative volume adjustment) tag
support to mpd, as a fallback if no replaygain tags are
found. The code is almost directly from madplay (GPL).

RVA2 tags are generated for example by the "normalize" utility.

Updated by: Avuton Olrich <avuton@gmail.com>

The input_stream object should only be closed by the MPD core
(i.e. decoder_thread.c / decoder_run()).  A decoder plugin which
attempts to close it will result in a segmentation fault.

The old code casted it to a 32 bit integer, which cut off bits.
AVFormatContext.duration is a int64_t, so use this type.

Use NULL instead.  Found by sparse.

Add G_GNUC_UNUSED attributes.

Variables which hold one of the DECODE_* values should be declared as
"enum mp3_action" instead of "int".

Use GLib's g_warning(), ... instead of MPD's deprecated log.h.

SEEK_SET is defined by unistd.h.  Explicitly include it.