Fixed CellFromPixel call when dragging an external item.

Author: aletorrado

src/gridstack.js

@@ -708,7 +708,7 @@
             var onDrag = function(event, ui) {
                 var el = draggingElement;
                 var node = el.data('_gridstack_node');
-                var pos = self.getCellFromPixel(ui.offset, true);
+                var pos = self.getCellFromPixel({left: event.pageX, top: event.pageY}, true);
                 var x = Math.max(0, pos.x);
                 var y = Math.max(0, pos.y);
                 if (!node._added) {