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.

For simpler seeking within current song.

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

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

Let the codec start with fresh buffers.  This should fix the remaining
seeking issues.

Use AVStream.time_base to convert the decoder_seek_where() value, and
pass the current stream number to av_seek_frame().

Use flag AV_TIME_BASE.

Code simplification.

av_open_input_stream() has been deprecated.

AVMetadata has been deprecated.

It's a no-op and deprecated.

avcodec_decode_audio3() has been added in libavformat 52.25.0, and the
predecessor avcodec_decode_audio2() has been deprecated.

fixes build with lavc 53.

For ffmpeg < 0.5.  Copied from libavutil 0.5.

avcodec_decode_audio3() has been added in libavformat 52.25.0, and the
predecessor avcodec_decode_audio2() has been deprecated.

All modern distros ship 0.5, so there's no need to support old and buggy
ffmpeg versions.

.. instead of av_get_bits_per_sample_format().  The SampleFormat enum
value is authoritative.

libavformat 0.6 does not pass the original URI pointer to the "open"
method, which leads to a crash because MPD was using a dirty hack to
pass a pointer to that method.

This patch switches to av_open_input_stream() with a custom
ByteIOContext class, instead of doing the URI string hack with
av_open_input_file().

Loosely based on a patch from Jasper St. Pierre.