Pass video size as codec metadata

On initial connection, scrcpy sent some device metadata:
 - the device name (to be used as window title)
 - the initial video size (before any frame or even SPS/PPS)

But it is better to provide the initial video size as part as the video
stream, so that it can be demuxed and exposed via AVCodecContext to
sinks.

This avoids to pass an explicit "initial frame size" for the screen, the
recorder and the v4l2 sink.