downscale jpeg and webp for html5 client

[totaam]

nvjpeg #2984 is re-using part of the video encoders code path, but without the csc or scaling part.

The video scaling is only calculated when evaluating pipelines, but we don't use pipelines for nvjpeg.
Also, we may want to use plain jpeg too.

[totaam]

Done as part of 749598a for nvjpeg, #3367 amongst many encoding improvements.
See also #3337, "pseudo-lossless": 51b0615.
jpeg and webp are now peudo video encoders: 104cfc4, 104cfc4.
Other scaling changes: 8302076.