Wait for keyframe after a video frame lost to minimize video distortion (thanks Faba Aa for the feedback)

[nanang]

As described ​here:

When one of video P frames lost, pjsip will continue decoding 
following  P frames, and will result in video distortion, this is 
the H.264's recommended action, but in real product, distortion not 
acceptable, drop distortion frames, wait for next keyframe is the 
standard action in most video products.

Perhaps also worth to note/check that in the start-up stage, the video stream should just keep returning 'no-frame' to application (e.g: renderer) until it gets a successful keyframe decoding.