Auto merge of #17467 - mantaroh:svg-zero-value, r=birtles

Add get_zero_value for IntermediateSVGPaint.

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This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1371199

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- [X] `./mach test-tidy` does not report any errors

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There are tests for these changes, a test case will be landed in reftests in https://bugzilla.mozilla.org/show_bug.cgi?id=1371199

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

@@ -2765,7 +2765,7 @@ impl Animatable for IntermediateRGBA {
 
     #[inline]
     fn get_zero_value(&self) -> Option<Self> {
-        Some(IntermediateRGBA::new(0., 0., 0., 1.))
+        Some(IntermediateRGBA::transparent())
     }
 
     #[inline]
@@ -2982,6 +2982,14 @@ impl Animatable for IntermediateSVGPaint {
         Ok(self.kind.compute_squared_distance(&other.kind)? +
             self.fallback.compute_squared_distance(&other.fallback)?)
     }
+
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> {
+        Some(IntermediateSVGPaint {
+            kind: option_try!(self.kind.get_zero_value()),
+            fallback: self.fallback.and_then(|v| v.get_zero_value()),
+        })
+    }
 }
 
 impl Animatable for IntermediateSVGPaintKind {
@@ -3012,6 +3020,18 @@ impl Animatable for IntermediateSVGPaintKind {
             _ => Err(())
         }
     }
+
+    #[inline]
+    fn get_zero_value(&self) -> Option<Self> {
+        match self {
+            &SVGPaintKind::Color(ref color) => color.get_zero_value()
+                                                    .map(SVGPaintKind::Color),
+            &SVGPaintKind::None |
+            &SVGPaintKind::ContextFill |
+            &SVGPaintKind::ContextStroke =>  Some(self.clone()),
+            _ => None,
+        }
+    }
 }
 
 #[derive(Copy, Clone, Debug, PartialEq)]