Support Hardware Transcoding in Shinobi, at least for vaapi.

This corrects support for hardware video input decoding, scaling, and monitor reencoding. The current example deals with using vaapi to decode four 4k h.265 streams and scale and re-enode them to 1080p h.265 on a Kaby Lake processor. Without it, -scale command line arguments are sent to ffmpeg, and there is no mechanism to keep the original decoded frames in a GPU surface for hardware reencoding. The wrong ffmpeg command line argument are generated.

This add a HWFormat option to set the output format. When it is non-blank (vaapi is specified in the current case), --scale is removed and --_scale is provided in its place.