Use more resiliant shadowroot checking

Also avoid cases where shadowRoot.elementFromPoint returns null
Fixes #1574 [0.9] Gesture event throws exception when dragged outside document

Author: dfreedm

src/standard/gestures.html

@@ -97,18 +97,18 @@
     return ta;
   }
 
-  function deepTargetFind(x, y) {
-    var node = document.elementFromPoint(x, y);
-    // this code path is only taken when native ShadowDOM is used
-    var next = node.shadowRoot;
-    while(next) {
-      next = next.elementFromPoint(x, y);
-      if (next) {
-        node = next;
-        next = next.shadowRoot;
-      }
+  function deepTargetFind(x, y, root) {
+    root = root || document;
+    var node = root.elementFromPoint(x, y);
+    var sr;
+    if (node) {
+      sr = node.shadowRoot;
     }
-    return node;
+    // if there is not a shadowroot, we are done
+    // if there is a shadowroot, it may have a node at x/y
+    // if there is a node at x/y in the shadowroot, return it
+    // otherwise, return this node
+    return sr ? (deepTargetFind(x, y, sr) || node) : node;
   }
 
   var Gestures = {