Use '-vcodec copy' on 'h264' video streams

Currently cast-localvideo tries to transcode everything to a common denominator.
This isn't very efficient when our input source is H.264@720p, for example. On
my machine, transcoding is slower than realtime, and my video ends up pausing
every 2 seconds as the transcoder catches up.

https://developers.google.com/cast/docs/media says that (at last some versions
of) H.264 should work just fine, so when the input file has an 'h264' video
stream, we use the 'copy' target codec.
merged acreefoot-video-passthru