refactor(zoomscroll): use layerX/Y instead more sophisticated solution.

As long as we use a container with relative positioning this should work.

close #83

lib/features/zoomscroll/ZoomScroll.js

@@ -56,8 +56,8 @@ function ZoomScroll(events, canvas) {
         // Gecko Browser should use _offsetX
         if(!event.originalEvent.offsetX) {
           offset = {
-            x: event.originalEvent._offsetX,
-            y: event.originalEvent._offsetY
+            x: event.originalEvent.layerX,
+            y: event.originalEvent.layerY
           };
         } else {
           offset = {
@@ -88,30 +88,3 @@ ZoomScroll.$inject = [ 'eventBus', 'canvas' ];
 
 module.exports = ZoomScroll;
 
-// Thx to https://bugzilla.mozilla.org/show_bug.cgi?id=69787#c37
-// for providing a work around for missing offsetX in Gecko
-Object.defineProperties(MouseEvent.prototype, {
-  _offsetRelativeElement: {
-    get: function() {
-      var element = this.target;
-      while (['block', 'inline-block', 'list-item', 'table', 'inline-table', 'table-caption',
-        'table-column', 'table-colgroup', 'table-header-group', 'table-row-group', 'table-footer-group',
-        'table-row', 'table-cell'].indexOf(window.getComputedStyle(element).display) === -1) {
-
-        element = element.parentNode;
-      }
-
-      return element;
-    }
-  },
-  _offsetX: {
-    get: function() {
-      return this.clientX - this._offsetRelativeElement.getBoundingClientRect().left;
-    }
-  },
-  _offsetY: {
-    get: function() {
-      return this.clientY - this._offsetRelativeElement.getBoundingClientRect().top;
-    }
-  }
-});