H.264 levels higher than 4.1 aren't supported by TiVo hardware; detect and transcode if needed

This patch requires the separate ffprobe program distributed with ffmpeg to be present; if ffprobe is not available, it will assume all H.264 streams are supported (the previous behavior).

It's possible Roamio hardware supports more advanced H.264 streams; I only have Premiere hardware to test with here.