Firefox, at least, supports playing `matroska/audio` containers natively but
the client was not checking for support. Clients that do not support
playing `matroska/audio` containers will fallback to transcoding.
Matroska audio containers (aka mka files) with Opus codec streams inside
were unplayable on the desktop client because hls.js was unable to
decode the stream, resulting in an infinitely "spinning" play button.
When configuring a stream, we now check for the opus codec and force AAC
transcoding.
Matroska containers support other codecs besides Opus, eg: mp3, which do
not require transcoding and work fine before this patch, which is why we
check for opus in codecsToForceAAC instead of AudioMimeType.MKA in
mimeTypesToForceAAC.
The AudioMimeType.OPUS mimetype is already marked as requiring
transcoding but since its inside a container this check does not
evaluate to true, we must check the codec explicitly.
Oliver Marriott