Switch to decode_audio4 when indexing audio
Add support for dumping planar audio while indexing
Use decode_audio4 in the audio sources
Interleave planar audio formats when decoding
Add support for resampling audio with libavresample

This currently has a bunch of limitations. The resampling can't be
reconfigured after the first call to GetAudio, and sample rate
conversions aren't supported.
Update changelog
Set the channel layout to the default if lavc doesn't set it