Bug 938686 - Make OggReader::DownmixToStereo public static. r=rillian

We want this to be available to the WebM reader as well.
Bug 938686 - Refactor Opus header parsing so it can be reused. r=kinetik

We want to be able to use the same header parser for both the WebM
and Ogg readers.
Bug 938686 - Support Opus in WebM. r=kinetik

Support the Opus audio codec in the WebM (Matroska) container.
This is part of the "WebM 2" proposed spec, which also includes
the new VP9 video codec. Alas we weren't able to get concensus
to change the doctype of filename extension to mark the revision
allowing the new codecs.