Consider adding support for k-SVC encoding in `vp9enc`

`vp8enc` has support for `--temp-svc` (following [from this command line argument](https://github.com/intel/libva-utils/blob/master/encode/vp8enc.c#L1001)).

http://issuetracker.google.com/issues/191203129 tracks launching VP9 k-SVC HW encoding on Gen 11. It'd be great to extend `vp9enc` to verify that the driver can support SVC modes (which includes, but is not restricted to, encoding using different resolutions reference frames).