Fix .shared classing

Now that Graph#parentWays is cached, the specialized Graph#parentStructure
method is no longer necessary.

This commit also demonstrates that it's relatively easy to write specs for
rendering now!

test/index_packaged.html

@@ -57,6 +57,7 @@
     <script src="spec/renderer/hash.js"></script>
     <script src="spec/renderer/map.js"></script>
 
+    <script src="spec/svg/vertices.js"></script>
     <script src="spec/svg/tag_classes.js"></script>
 
     <script src="spec/ui/inspector.js"></script>