[Galene] Fixes to VP8 parsing

[Galene] Fixes to VP8 parsing

[Juliusz Chroboczek]

I've just merged a fix to VP8 parsing into Galene master.  Thanks to Jake
for his help with debugging the issue.

As far as I can tell, the issue only occurs with streams generated by
GStreamer, where the symptom manifests as occasional stuttering of the
video (the video freezes and recovers at the next keyframe).  However,
I suppose it also occurs with other software that performs sloppy
packetisation of VP8 streams, so if you've been witnessing stuttering,
please check Galene master and let me know if it fixes it.  I'm
particularly interested to know whether it fixes the issues with Safari
screensharing.

In case you're interested in the technical details, the full explanation
is here:

  https://github.com/pion/rtp/commit/8516abc948daffecb28fffb72d23bc08182bbfe0

(Aside: the VP9 packetisation rules are way cleaner than the ones for VP8,
VP9 is generally less hackish than VP8, and consequently Galene's support
for VP9 is fairly solid.  Unfortunately, VP9 is not supported by Safari,
and it has some bugs in older versions of Firefox.  It is therefore not
yet reasonable to make VP9 the default in Galene, even though VP9 is
almost ten years old.)

-- Juliusz "waiting for AV2"