made it like a codec mod so we can share the same code in case opus is fully supported in webm
add -lvorbis -lvorbisenc to Makefile
the audio is still completely silence, hope we can find out why
use SWITCH_RTP_MAX_BUF_LEN for video
timestamp has to times 1000 or it too short
neither VLC nor Chrome plays it because they don't support OPUS,
but you can use `ffmpeg -i a.webm b.webm`