[css-easings-2] `linear()` serialization does not conform to shortest principle

[cdoublev]

https://drafts.csswg.org/css-easing-2/#linear-easing-function-serializing

The serialization of linear() includes input values for each point, and input values are never less than the input of the previous point.

For example:

• linear(0, 0.25, 1) serializes as linear(0 0%, 0.25 50%, 1 100%)
• linear(0 20%, 0.5 10%, 1) serializes as linear(0 20%, 0.5 20%, 1 100%)
• linear(0, 0.25 25% 75%, 1) serializes as linear(0 0%, 0.25 25%, 0.25 75%, 1 100%)

https://drafts.csswg.org/cssom-1/#serialize-a-css-value

If component values can be omitted or replaced with a shorter representation without changing the meaning of the value, omit/replace them.

It should probably be consistent with the serialization of a gradient color stop list (related).

[tabatkins]

I've adjusted the serialization algo to match what impls appear to do for gradients - expand double-% stops into two stops, but preserve omitted %s in the serialization.

I'll likely need to adjust WPTs to match, so keeping this open.

[cdoublev]

3. Otherwise, append " " (U+0020 SPACE) to s, then serialize the control point’s output progress value as a <percentage> and append it to s.

I think it should be "input progress value" at this step of serialize a linear() control point.

[tabatkins]

Thanks for the catch, fixed.