Auto merge of #13157 - Manishearth:background-size, r=emilio

stylo: support background-size and corner gradients

Forgot to fix this in the midst of all other things fixed in #12945

<s>(not yet tested, need to figure out how to test bg-size with gradients)</s>

r? @bholley

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ports/geckolib/gecko_bindings/sugar/ns_style_coord.rs

@@ -40,6 +40,16 @@ impl CoordDataMut for nsStyleCoord {
     }
 }
 
+impl nsStyleCoord_CalcValue {
+    pub fn new() -> Self {
+        nsStyleCoord_CalcValue {
+            mLength: 0,
+            mPercent: 0.0,
+            mHasPercent: false,
+        }
+    }
+}
+
 impl nsStyleSides {
     #[inline]
     pub fn data_at(&self, index: usize) -> SidesData {