Rollup merge of #17013 - mbrubeck:stylo, r=emilio

Bug 1363639 - stylo: Make interpolation of rect() match Gecko

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components/style/properties/helpers/animated_properties.mako.rs

@@ -753,6 +753,7 @@ impl <T> Animatable for Option<T>
             (&Some(ref this), &Some(ref other)) => {
                 Ok(this.add_weighted(other, self_portion, other_portion).ok())
             }
+            (&None, &None) => Ok(None),
             _ => Err(()),
         }
     }
@@ -763,6 +764,7 @@ impl <T> Animatable for Option<T>
             (&Some(ref this), &Some(ref other)) => {
                 this.compute_distance(other)
             },
+            (&None, &None) => Ok(0.0),
             _ => Err(()),
         }
     }