[css-color-adjust-1] color-adjust initial value should be auto

[fantasai]

To make it easier to extend color-adjust to other use cases, I think it makes sense to make the initial value auto and have it compute to different values depending on what adjustment is being performed. E.g. in print mode, it might compute to economy, but on a typical screen it will always compute to exact.

(One extension we might consider is something to represent automatic color luminance inversions; this is not the same as economy, but could be reasonably represented in color-adjust if we make it extensible in this way. See discussion on Apple's color-filtering.)

[tabatkins]

(Closed after discussion with fantasai; her use-cases were addressed in other ways already.)