webrtcbin: Added fields to track media line index of data channel and advance media line...
Added fields to track media line index of data channel and advance media line index of trasscievers added after data channel.
This fixes issue #1377 (closed) (I forgot to add it to the commit message).
The gist of this patch is this: When a data channel is added (this covers the offer scenario; I haven't worked through the answer scenario) and SDP successfully negotiated, advance the mline index (sink pad index) so that new transceivers created after the data channel and sdp negotiation don't conflict with the same media line as the data channel.
I'm actively using this patch in local development to allow continued renegotiation after a data channel has been added.
And sorry for any formatting violations or convention violations; C is not my first language.