Update webrtctransceiver example to use multiple streams

Submitted by Andreas Frisch @fraxinas

Link to original bug (#796387)

Description

Created attachment 372389
modified example (adds an audio stream and checks caps)

I've noticed that my recvonly webrtcbin application breaks once I enabled the new ULPFEC/RED features.

it's really easy to reproduce with this modified example of webrtctransceiver.c
it works when commenting out the ULPFEC/RED features. if they're enabled, the pad-added callback asserts because the pad's CAPS aren't fully set.

Attachment 372389, "modified example (adds an audio stream and checks caps)":
webrtctransceiver.c