Output per-frame metrics

[shssoichiro]

This has been requested for AWCY, probably useful for other purposes as well.

The API exists, just needs CLI support. Should be controlled by a CLI flag (off by default).

[figgis]

Could this feature get some love? This would solve so many issues for me and I can stop using my own tools to get the values per frame. Among other thing I use PSNR/SSIM to track frame differences where my video algos fails, these failures are not visible when looking at the average values.

[figgis]

ping