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.