Fix drag behavior on for <svg:use> elements

Drag behavior is broken for <svg:use> elements on certain
browsers. This includes d3.layout.force(...).drag()

On browsers like Internet Explorer 11, the <svg:use>
EventTarget is represented by a SVGElementInstance that looks
like this:

    [object SVGElementInstance]
       {
          [functions]: ,
          __proto__: { },
          childNodes: { },
          constructor: { },
          correspondingElement: { },
          correspondingUseElement: { },
          firstChild: null,
          lastChild: null,
          nextSibling: null,
          parentNode: { },
          previousSibling: { }
       }

No other properties such as .style are present. We must use the
correspondingElement property to access the actual SVG element
instantiated by the <svg:use> in order to have a successful drag.

Author: stefwalter

d3.js

@@ -1204,7 +1204,7 @@
     }
     function dragstart(id, position, subject, move, end) {
       return function() {
-        var that = this, target = d3.event.target, parent = that.parentNode, dispatch = event.of(that, arguments), dragged = 0, dragId = id(), dragName = ".drag" + (dragId == null ? "" : "-" + dragId), dragOffset, dragSubject = d3.select(subject(target)).on(move + dragName, moved).on(end + dragName, ended), dragRestore = d3_event_dragSuppress(target), position0 = position(parent, dragId);
+        var that = this, target = d3.event.target.correspondingElement || d3.event.target, parent = that.parentNode, dispatch = event.of(that, arguments), dragged = 0, dragId = id(), dragName = ".drag" + (dragId == null ? "" : "-" + dragId), dragOffset, dragSubject = d3.select(subject(target)).on(move + dragName, moved).on(end + dragName, ended), dragRestore = d3_event_dragSuppress(target), position0 = position(parent, dragId);
         if (origin) {
           dragOffset = origin.apply(that, arguments);
           dragOffset = [ dragOffset.x - position0[0], dragOffset.y - position0[1] ];