Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

FfmpegDecoderPlugin: Allocate the interleaved buffer as needed instead of using an arbitrary big constant.

Also, save a memcpy when FFmpeg outputs interleaved audio.

Fixes build with FFmpeg 2.0.

Now that even Debian stable has picked up version 0.8, we can safely
make this a requirement, and remove a lot of old cruft.

Only relevant for old ffmpeg versions.

AVFrame must be allocated with avcodec_alloc_frame().

This is the version present in Ubuntu Oneiric, the oldest distribution
with gcc 4.6.  Debian Squeeze is off target, because it has gcc 4.4,
which is unable to compile MPD anyway.

This commit drops all API compatibility hacks for older versions.

Hide the definition from C code, to prepare the transition to C++.

Implements Mantis feature request 3582.

Interleaving was completely wrong.  This code was never used, so it
didn't have an effect.

Works around assertion failure in decoder_timestamp().

Use av_get_sample_fmt_string() to obtain a human-readable string.

API simplification.

"Frame skipped" might cause the impression that the decoding of a whole
song failed.

This was disabled when compiled with a new ffmpeg version.  Older
ffmpeg versions used it explicitly, while newer ones may pass it
through from the codec.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

All supported ffmpeg/libav versions have this.

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().