Fix the incrmental reflow behavior of text-overflow
This patch allows Servo to incrementally reflow truncated fragments correctly.
* The untruncated version of a fragment is preserved, and when incrementally reflowing, the untruncated version is what gets reflowed. If it needs truncated, it will get truncated again.
* The ellipsis fragments are skipped when incrementally reflowing a line. If it is still needed, it will be recreated.
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#14952
- [X] There are tests for these changes OR
<!-- 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/14989)
<!-- Reviewable:end -->
6a04aea4a5
