[css-images-4] Fix gradient color stop syntax

[AmeliaBR]

Closes #1334

I'm pretty sure this was the only change required. Not sure why I didn't make a PR at the time I filed the issue, but I guess I was busy…

[fantasai]

While we're here... why is the color and length reorderable in Level 4? There was a fixed order in Level 3, was that a change we decided to make or an error in the spec?

[AmeliaBR]

@fantasai That's a good question. I just tested it in Chrome, Edge, and Firefox, and none support the length value before the color.

The Git blame isn't helping me find a reason, either. The && syntax goes back to the first commit for Images 4.

As an author, I wouldn't mind having the extra flexibility. I've made the mistake of writing it in the wrong order & then having to debug why my gradient wasn't showing up.

But we'd need commitments to actually implement it — maybe at the same time as implementing the two-value syntax? But then we'd need to discuss whether blue, 20px green 30px, blue is a valid two-value form.

So maybe that's a separate issue?