Auto merge of #11873 - notriddle:rotate_anim, r=glennw

Rotate animation

Add rotation interpolation code for the case where their direction vectors are equal.
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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 #11808 (github issue number if applicable).
- [X] There are tests for these changes

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -4540,6 +4540,18 @@
             "url": "/_mozilla/css/root_pseudo_a.html"
           }
         ],
+        "css/rotate_anim.html": [
+          {
+            "path": "css/rotate_anim.html",
+            "references": [
+              [
+                "/_mozilla/css/rotate_anim_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/rotate_anim.html"
+          }
+        ],
         "css/rtl_body.html": [
           {
             "path": "css/rtl_body.html",
@@ -11696,6 +11708,18 @@
           "url": "/_mozilla/css/root_pseudo_a.html"
         }
       ],
+      "css/rotate_anim.html": [
+        {
+          "path": "css/rotate_anim.html",
+          "references": [
+            [
+              "/_mozilla/css/rotate_anim_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/rotate_anim.html"
+        }
+      ],
       "css/rtl_body.html": [
         {
           "path": "css/rtl_body.html",