Wrap songvec_find() and other songvec methods.

Random access is not needed, and a linked list is easier to manage: we
don't need to (re-)allocate the pointer array.

Pass only the "name" to a directory, instead of the full (relative)
path.

This overhead is completely useless, because no caller expects this
behaviour.

The given song must exist, or MPD will abort.  Calling it with an
unknown song is illegal.

Conflicts:
	src/decoder/ffmpeg_decoder_plugin.c

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.

Use libasound's polling functions, implement a bridge to GSource /
GPollFD and send idle events to clients when an external program
changes the ALSA mixer volume.

Better error handling.

Conflicts:
	src/decoder/ffmpeg_decoder_plugin.c
	src/input/ffmpeg_input_plugin.c

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.

Keep the device open as long as the output is enabled, but initialize
it only when playback starts.

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.

For simpler seeking within current song.