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

style: Do not immediately convert absolute specified lengths

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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 :)

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15729.

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- [X] There are tests for these changes

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