Tunnel source event preventDefault to PointerEvents

- Only way to disable some default actions
- Related to #103

src/dispatcher.js

@@ -223,6 +223,7 @@
         inEvent.relatedTarget = null;
       }
       var e = new PointerEvent(inType, inEvent);
+      e.preventDefault = inEvent.preventDefault;
       this.targets.set(e, this.targets.get(inEvent) || inEvent.target);
       return e;
     },
@@ -244,6 +245,12 @@
         p = CLONE_PROPS[i];
         eventCopy[p] = inEvent[p] || CLONE_DEFAULTS[i];
       }
+      // keep the semantics of preventDefault
+      if (inEvent.preventDefault) {
+        eventCopy.preventDefault = function() {
+          inEvent.preventDefault();
+        };
+      }
       return eventCopy;
     },
     getTarget: function(inEvent) {

src/mouse.js

@@ -42,6 +42,12 @@
     },
     prepareEvent: function(inEvent) {
       var e = dispatcher.cloneEvent(inEvent);
+      // forward mouse preventDefault
+      var pd = e.preventDefault;
+      e.preventDefault = function() {
+        inEvent.preventDefault();
+        pd();
+      };
       e.pointerId = this.POINTER_ID;
       e.isPrimary = true;
       e.pointerType = this.POINTER_TYPE;

src/touch.js

@@ -154,6 +154,16 @@
     processTouches: function(inEvent, inFunction) {
       var tl = inEvent.changedTouches;
       var pointers = touchMap(tl, this.touchToPointer, this);
+      // forward touch preventDefaults
+      pointers.forEach(function(p) {
+        var pd = p.preventDefault;
+        p.preventDefault = function() {
+          this.scrolling = false;
+          this.firstXY = null;
+          inEvent.preventDefault();
+          pd();
+        };
+      }, this);
       pointers.forEach(inFunction, this);
     },
     // For single axis scrollers, determines whether the element should emit