WIP: Add audio sink for Magic Leap platform
This is a follow-up MR to !638 (merged) where we are adding Media Codec support.
mlaudiosink is a new audio sink based on GstAudioSink using Magic Leap's SDK API. While working on this, it raised questions on how to use GstAudioSink base class, that I reported here: gst-plugins-base#665. Setting WIP until those questions are answered.