scroll offset values should use length-percentage type to allow calc()

[AmeliaBR]

The startScrollOffset and endScrollOffset points on a scroll timeline are currently defined with the following CSS grammar:

auto | <length> | <percentage>

That should be

auto | <length-percentage>

in order to allow calc() expressions combining lengths and percentages, such as calc(100% - 5rem).

Same for the <scroll-offset> part of the scroll() function definition.

[majido]

Thanks for the suggestion. The above spec change should fix this.

Note we already have a test to verify length and percentages can be used in a calc for these offsets.

/cc @andruud FYI.