Auto merge of #16229 - tomhoule:fix-lengths, r=emilio

style: Do not immediately convert absolute specified lengths

<!-- Please describe your changes on the following line: -->

This PR aims to solve issue #15729. I tried to follow the recommendations there as much as possible.

This is my first attempt at contributing to Servo, so this will probably need a lot of input, although I'm eager to make it as polished as possible.

- The base inaccuracy issue seems solved, as can be easily verified with the `console.log` based example in the issue.
- Very basic unit tests were added.

I have doubts mainly about the right way to represent these new enum variants for the various length units:

1. With new enum variants in `NoCalcLength` *and* newtypes (current solution)
2. With a `NoCalcLength::Absolute` variant that contains a new `AbsoluteLength` enum, but without newtypes
3. Same as solution 2 but with newtypes

- I mostly cared about unit tests until now but will investigate other types of tests
- Tests to check the clamping
- Write a proper commit message

Thanks for your time and feedback :)

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15729.

<!-- Either: -->
- [X] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/16229)
<!-- Reviewable:end -->

tests/unit/style/properties/viewport.rs

@@ -6,7 +6,7 @@ use app_units::Au;
 use style::properties::PropertyDeclaration;
 use style::properties::longhands::border_top_width;
 use style::values::HasViewportPercentage;
-use style::values::specified::{Length, NoCalcLength, ViewportPercentageLength};
+use style::values::specified::{AbsoluteLength, Length, NoCalcLength, ViewportPercentageLength};
 
 #[test]
 fn has_viewport_percentage_for_specified_value() {
@@ -20,7 +20,7 @@ fn has_viewport_percentage_for_specified_value() {
 
     let pabs = PropertyDeclaration::BorderTopWidth(Box::new(
         border_top_width::SpecifiedValue::from_length(
-            Length::NoCalc(NoCalcLength::Absolute(Au(100)))
+            Length::NoCalc(NoCalcLength::Absolute(AbsoluteLength::Px(Au(100).to_f32_px())))
         )
     ));
     assert!(!pabs.has_viewport_percentage());