It might be possible that if we set webrtcbin to the NULL state some
tasks (idle sources) are still executed and they might even freeze. The freeze
is caused because the webrtcbin tasks don't hold a reference to webrtcbin and
if it's last unref inside the idle source itself this will not allow the main
loop to finish because the main loop is waiting on the idle source to finish.

We now start and stop webrtcbin thread when changing states. This will allow
the idle sources to finish properly.

https://bugzilla.gnome.org/show_bug.cgi?id=797251

Mostly follows the W3C specification
https://www.w3.org/TR/webrtc/#peer-to-peer-data-api

With contributions from:
Mathieu Duponchelle <mathieu@centricular.com>

https://bugzilla.gnome.org/show_bug.cgi?id=794351

It is possible and often desirable to pass multiple ICE relays
to libnice agents, the "turn-server" property, while convenient
to use from the command line, does not allow that.

This adds a new action signal, "add-turn-server" to address that.

https://bugzilla.gnome.org/show_bug.cgi?id=797012

When it parses SDP, it doesn't free the error object.

https://bugzilla.gnome.org/show_bug.cgi?id=796830

When generating caps with no ssrc, at least throw a
warning instead of using an uninitialised stack variable

https://bugzilla.gnome.org/show_bug.cgi?id=796810

Fix a leaked string when building RTX info.

When negotiation is triggered by receiving caps on our sink pad
probes, we could encounter a race condition where need-negotiation
is emitted and the application requires the creation of an offer
before the current caps were actually updated.

This led to retrieving incomplete caps when creating the offer,
using find_codec_preferences -> pad_get_current_caps.

Instead, as we save the caps in the probe callback anyway, it is better
and thread safe to use these if they were set.

https://bugzilla.gnome.org/show_bug.cgi?id=796801

Fixes random crashes when an allocated webrtcbin isn't
given fresh 0-filled memory in its allocation. It works
mostly because GMutex and GCond are automatically initialised
in that case.

This lets users call gst_pad_get_current_caps on newly-added
pads to easily determine what to plug them into.

We cannot copy sticky events unconditionally in core,
see #719437

https://bugzilla.gnome.org/show_bug.cgi?id=796387

https://bugzilla.gnome.org/show_bug.cgi?id=796429

https://bugzilla.gnome.org/show_bug.cgi?id=795044

Fixes ffeb09e4

if (sscanf(...)) {  // != 0
  error;
}

Is not correct where != 0 indicates some kind of success.

Check instead that the correct number of elements were slurped.

CID #1429140

If we fail parsing rtpbin pad names, someone has screwed up so critical
and return.

CID #1429142

SDP's are generated and consumed according to the W3C PeerConnection API
available from https://www.w3.org/TR/webrtc/

The SDP is either created initially from the connected
sink pads/attached transceivers as in the case of generating an offer or
intersected with the connected sink pads/attached transceivers as in
the case for creating an answer.  In both cases, the rtp payloaded streams
sent by the peer are exposed as separate src pads.

The implementation supports trickle ICE, RTCP muxing, reduced size RTCP.

With contributions from:
Nirbheek Chauhan <nirbheek@centricular.com>
Mathieu Duponchelle <mathieu@centricular.com>
Edward Hervey <edward@centricular.com>

https://bugzilla.gnome.org/show_bug.cgi?id=792523