From the spec:
> This property previously accepted the values optimizeSpeed and
optimizeQuality. These are now deprecated; a user agent must accept
them as valid values but must treat them as having the same behavior
as pixelated and smooth respectively, and authors must not use them.
- https://www.w3.org/TR/css-images-3/#the-image-rendering
Alias values are represented by "alias-name=real-name".
We have a lot of repetitive code for converting between ValueID and
property-specific enums. Let's see if we can generate it. :^)
This first step just produces the enums, from a JSON file. The values in
there are a duplication of what's in Properties.json, but eventually
those will go away.
