fix performance issue in Leaf (#158)

lib/components/leaf.js

@@ -127,10 +127,11 @@ class Leaf extends React.Component {
     })
 
     const style = marks.reduce((memo, mark) => {
-      return {
-        ...memo,
-        ...renderMark(mark, marks),
-      }
+      const renderedMark = renderMark(mark, marks)
+      Object.keys(renderedMark).forEach((k) => {
+        memo[k] = renderedMark[k]
+      })
+      return memo
     }, {})
 
     return (