[Galene] Alternative codecs

[Juliusz Chroboczek <jch@irif.fr>]

I've just pushed code to deal with codecs other than VP8 and Opus in
Galène.  It's in branch "codecs".  Be aware that I'll be rebasing this
branch, you might get git conflicts if you track it.

After you've rebuilt Galène, you will be able to specify something like

  "codecs": ["h264", "ulaw"]

The default is unchanged -- it's still ["vp8", "opus"].  If multiple
codecs of the same type (audio and video) are specified, the sender
(presenter) will choose the codec; if the receiver cannot grok the codec,
they'll receive a big red error and the video will not appear.

Supported codecs are:

  Video: vp8 (all browsers)
         vp9 (Chrome/Chromium/Firefox/Brave)
         h264 (anything except Chromium and Firefox under Debian, who
               violate the WebRTC spec and are as cool about it as the
               Mairie de Paris is about breaking the law)

  Audio: opus (all browsers)
         g722 (all browsers?)
         ulaw, alaw (giggle)

Limitations: saving to disk is only implemented for VP8/Opus (you'll get
a big yellow warning if you try to save anything else).  The support for
VP9 is not quite complete yet (I don't know how to detect a keyframe in
VP9, so the keyframe optimisations are disabled).

Unfortunately, it's not as useful as I hoped:

 - h.264 would be useful to solve the problems of MacBooks overheating,
   unfortunately, Chromium and Firefox on Debian/Ubuntu don't implement
   h.264, in clear violation of the WebRTC spec;

 - VP9 would be useful if it were supported by Apple;

 - I know nothing about G.722, and in what circumstances it might be useful;

 - mu-law and A-law are only included for nostalgia's sake (and because it
   was easy once I had built the infrastructure).

-- Juliusz