Auto merge of #13331 - mrobinson:background-repeat, r=emilio

Add support for background-repeat: space and round

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Add support for new background-repeat modes and upgrade Euclid, which now has support for basic arithmetic with Size2D.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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This adds support for more background-repeat modes using the legacy
rasterization backend.

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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13331)
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tests/wpt/metadata-css/css-backgrounds-3_dev/html4/background-repeat-round.htm.ini

@@ -1,3 +0,0 @@
-[background-repeat-round.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css-backgrounds-3_dev/html4/background-repeat-space.htm.ini

@@ -1,3 +0,0 @@
-[background-repeat-space.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css-backgrounds-3_dev/html4/background-size-025.htm.ini

@@ -1,3 +0,0 @@
-[background-size-025.htm]
-  type: reftest
-  expected: FAIL