ts-jitterbuffer: set jbuf delay when instantiating it

The internal (C) jitterbuffer needs to know about the configured latency when calculating a PTS, as it otherwise may consider that the packet is too late, trigger a resync and cause the element to discard the packet altogether.

I could not identify when this was broken, but the net effect was that in the current state, ts-jitterbuffer was discarding up to half of all the incoming packets.