[Galene] Saving to disk reworked again

[Juliusz Chroboczek <jch@irif.fr>]

Hi,

I've just done some work on saving to disk.  I'll be testing myself, but
please feel free to test in the meantime.

Saving to disk was pretty broken before 0.5.1.  In 0.5.1, I managed to get
it to work reasonably well even with massive packet loss, but the result
was that we got pretty bad audio-video desynchronisation.

I've now implemented audio-video synchronisation almost correctly.  We
properly parse remote NTP timestamps, correlate them with RTP timestamps,
and use the data to compute MKV timestamps.

The only remaining issue is what happens before we receive NTP information
from the remote.  In that case, we use the reception timestamp, which
yields a desynchronisation on the order of 100-200ms in my experiments.
We resynchronise when the first sync packet arrives, but in many cases it
causes Matroska timestamps going backwards, which is illegal.

I've tried working around the issue by buffering packets until the first
sync timestamp, but I've found that the browsers only send audio sync
information after a couple of seconds, which means we need to buffer
hundreds of packets.  I've kept the code (see the branch "diskwriter-buffer"),
but haven't decided whether to merge it yet.

Please test!

-- Juliusz