avauddec: Add clipping meta support for gapless playback

This enables support for GstAudioClippingMeta in libav audio decoders.

Together with gst-plugins-good!344 (closed) , this allows for gapless mp3 playback with avdec_mp3 for example. Also, gapless AAC is possible if for instance qtdemux can be made to parse the "iTunSMPB" tag and add clipping meta accordingly. (For details about iTunSMPB, check out https://yabb.jriver.com/interact/index.php?topic=63215 .)