app_echo: forward and generate VIDUPDATE frames

When using app_echo via WebRTC with VP8 video the video would appear
only after a few minutes, because there would be nothing to request
a full reference frame.

This fixes the problem in both ways:
- echos any VIDUPDATE frames received on the channel
- sends one such frame when first video frame is to be forwarded

This makes the echo work with Firefox and Chrome WebRTC implementation.

ASTERISK-25867 #close

Change-Id: I73bda87bf7532ee8bfb28d917045a21034908c1e