[Galene] SVC support

[Juliusz Chroboczek <jch@irif.fr>]

Hi,

I've just pushed the code for Scalable Video Coding (SVC) to the branch
"svc" of the github repository; be aware that I will be rebasing this
branch.  It's currently running on galene.org port 8444 (but not port
8443, which is running stable 0.3.5).

SVC is a technique where a single video stream is "layered": frames are
labelled with information about whether they can be dropped, and the
server can reduce the bitrate sent to a client.  The advantage of SVC over
simulcast is that switching layers is a much more lightweight operation:
switching simulcast streams requires a keyframe, while switching scalable
layers can happen a few times per packet.

SVC requires no support on the receiver side -- it's the sender who does
the layering, the receiver receives a perfectly banal video stream.
Sending a scalable stream is controlled by the simulcast switch, due to
some limitations in both Chrome/Chromium and Firefox, so you cannot have
SVC without simulcast.  Which is a waste, but what can I do.

You can test by enabling simulcast then testing the "stats.html" page; the
layer chosen for each down track is indicated before the bitrate.

Temporal scalability (reducing the framerate) is supported for both VP8
and VP9; unfortunately, I wasn't able to test the VP9 support, since my
version of Chromium immediately crashes when you request a VP9 simulcast
stream.  Spacial scalability (reducing the resolution) should be doable
for VP9, but the point is somewhat moot until Chromium fix their bug.

-- Juliusz