[css-images] Added image-rendering:high-quality

Per WG resolution from the Feb 17 telcon, I added a "high-quality"
value to the image-rendering property:
<https://drafts.csswg.org/css-images/#valdef-image-rendering-high-quality>

The call purposely didn't settle on a name - is the group okay with
"high-quality"?

The group also mentioned having the old optimizeQuality value be an
alias for this.  After looking at the spec again, I think I don't want
to do this - the old SVG version of this property had only two values
- optimizeSpeed and optimizeQuality, with optimizeSpeed in practice
meaning "pixelate this", so the current mapping of
optimizeSpeed=>pixelated and optimizeQuality=>auto seems correct.

~TJ