Add support for WebM VP9/Opus playback and transcodes

Enables playback of WebM VP9/Opus and VP9/Vorbis files.

Adds transcode definitions for VP9/Opus transcodes at half the
video bitrate of VP8/Vorbis. HD transcodes use tiled encoding
mode which will help some decoders to parallelize playback.
Not yet enabled by default, so this should be safe to merge.

Requires ffmpeg or avconv with vp9 and opus support compiled in;
a suitable ffmpeg is available in Wikimedia Ubuntu repos as for
instane in MediaWiki-Vagrant.

To test, set up suitable ffmpeg & enable the transcodes manually:

$wgEnabledTranscodeSet[] = WebVideoTranscode::ENC_VP9_360P;
$wgEnabledTranscodeSet[] = WebVideoTranscode::ENC_VP9_480P;
$wgEnabledTranscodeSet[] = WebVideoTranscode::ENC_VP9_720P;
$wgEnabledTranscodeSet[] = WebVideoTranscode::ENC_VP9_1080P;

VP9 will take priority over VP8 if available and the browser
supports playback; works in Firefox 40 and Chrome 44.

Bug: T62272
Bug: T63805
Change-Id: Idbea47fbde2a8862084a7e77eb1ca928083d5b85