Fix memory issue with ClipPath

Fix memory issue with ClipPath.

lib/attributes.js

@@ -46,10 +46,7 @@ const NodeAttributes = {
     },
     opacity: true,
     clipRule: true,
-    clipPathRef: true,
-    clipPath: {
-        diff: arrayDiffer
-    },
+    clipPath: true,
     propList: {
         diff: arrayDiffer
     },

lib/extract/extractClipping.js

@@ -16,9 +16,7 @@ export default function (props) {
         let matched = clipPath.match(clipReg);
 
         if (matched) {
-            clippingProps.clipPathRef = matched[1];
-        } else {
-            clippingProps.clipPath = new SerializablePath(clipPath).toJSON();
+            clippingProps.clipPath = matched[1];
         }
     }