Immediately decode incoming frames to avoid loosing key frames when the jbuf is full and improve latency

[bennylp]

Image decoding is now done in on_rx_rtp() callback by the network thread as soon as full image is available in the jitter buffer.

The default behavior is to only decode one image and suspend further decoding until this image is consumed by get_frame(). The decoding can be made more aggressive by setting PJMEDIA_VID_STREAM_SKIP_PACKETS_TO_REDUCE_LATENCY to non-zero, and in this case the stream will always decode any available image in the jitter buffer regardless whether it will overwrite previously decoded image or not. This will notably improve latency.