Implements support for libavcodec 0.9, which removes the compatibility
macros SAMPLE_FMT_*

Minor optimisation.

Use the tag_item_names table to look up the names of all MPD tags, and
remove the duplicate entries from ffmpeg_tag_maps.

Allow using this function without the ffmpeg_tag_map struct.

Eliminate some duplicate code.

Move the #ifdefs out of _copy_metadata().

No interest in this return value.

Require libavutil 51.5.0.

This function was added when the libavformat version was 53.2.0, but
the actual release 53.2.0 did not have it.

Needed for av_rescale_q() in ffmpeg 0.8.

avcodec_open() has been deprecated.

It's a no-op and deprecated.

URLContext is deprecated.

Support ancient ffmpeg versions.

When we don't have enough data, generate some silence, hoping the
input buffer will fill soon.  Reducing the render buffer size is not
legal.

Blocking inside the render callback is forbidden, and this sleep call
didn't make any sense.

Moving songs using either 'move' or 'moveid' to position -1 (after the
current song) would fail for a song which is just before the current
song.
This patch corrects the check to see if the current song is in the range
to be moved. Since the range is from `start` up to `end` (exclusive) the
check was incorrect, but is now fixed.

The OpenAL specification says that AL_FORMAT_MONO8 and
AL_FORMAT_STEREO8 expect unsigned 8 bit samples, but MPD uses unsigned
samples.

The local variable was already divided by 1000, and the return value
was being divided by 1000 again - doh!  This caused delays in the
httpd output plugin that were too small by three orders of magnitude,
and the buffer was filled too quickly.

Replace -I with -isystem in GLIB_CFLAGS.

This workaround leads to an infinite loop instead of an assertion
failure, but hey, now it's libmp4ff's fault.

Initialize the audio_format before calling avcodec_open(), because
avcodec_open() will fill bogus values.

Yet another common support case.

This is a common support case, and hopefully, the new error message
will allow the user to understand the error without requiring support.

Use stat() instead of g_file_test() to detect other types of errors,
such as "permission denied".

Convert to padded 24 bit samples, instead of falling back to 16 bit.

This fixes a buffer corruption bug; pcm_buffer is not designed to be a
persistent buffers, and will discard anything between two consecutive
calls.