Make emtpy rule can be inserted into rule tree
<!-- Please describe your changes on the following line: -->
We really want to ensure empty rule nodes appear in the rule tree for devtools, this condition ensures that if we find an empty rule node, we insert it at the normal level.
---
<!-- 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 [Bug 1384802](https://bugzilla.mozilla.org/show_bug.cgi?id=1384802)
<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because of the test cases are in Gecko.
<!-- 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/18099)
<!-- Reviewable:end -->
Bindgen bitfield enums don't work as return values with the Linux 32-bit ABI at
the moment because they wrap the value in a struct.
This causes the Rust side to believe the callee expects space for the struct
return value, while C++ believes it's just an integer value.
MozReview-Commit-ID: FRBqlZuMiAR
Make content animatable
<!-- Please describe your changes on the following line: -->
---
<!-- 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
<!-- Either: -->
- [X] There are tests for these changes. The test codes are patch part 3 and 4 in https://bugzilla.mozilla.org/show_bug.cgi?id=1382136
<!-- 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/18085)
<!-- Reviewable:end -->
Don't convert zero component value to 'auto' in clone__moz_image_region.
Currently Gecko treats 'auto' component value as zero for animation.
<!-- Please describe your changes on the following line: -->
https://bugzilla.mozilla.org/show_bug.cgi?id=1387951
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
<!-- 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/18079)
<!-- Reviewable:end -->
For CSS Transitions we want this case to return Err() so we know that the two
values are not interpolable.
For CSS Animations/Web Animations we implement discrete animation as the
fallback behavior when Err() is returned.
column-count should be greater than or equal to 1. Also it should be less than
or equal to nsStyleColumn_kMaxColumnCount since we limit it in
set_column_count().
Make scroll-snap-{coordinate, destination} discrete animation.
<!-- Please describe your changes on the following line: -->
https://bugzilla.mozilla.org/show_bug.cgi?id=1387941
---
<!-- 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
<!-- 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/18069)
<!-- Reviewable:end -->
Make -moz-appearance animatable
<!-- Please describe your changes on the following line: -->
---
<!-- 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
<!-- Either: -->
- [X] There are tests for these changes. The test code is patch part 2 in https://bugzilla.mozilla.org/show_bug.cgi?id=1382138
<!-- 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/18066)
<!-- Reviewable:end -->
We convert a perspective function into a ComputedMatrix, but we apply the
perspective length to a wrong matrix item. We should put it into m34,
instead of m43. m43 is for translate parameter Z.
Introduce and derive ComputeSquaredDistance
This new trait merges the former `Animatable` methods `compute_distance` and `compute_squared_distance`.
<!-- 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/18058)
<!-- Reviewable:end -->
