Use fabrics getPointer in mouse events

Hi Matthieu,

there was an issue with `x/y` calculation in mouse event handlers of the crop plugin. `pageX/pageY` was used with  fabrics `_offset.left/_offset.top`. It becomes sometimes invalid, e.g. if any canvas ancestor is fixed positioned. Better to use built in tool for those calculations - `canvas.getPointer`.  Now cropping area has proper position in fixed containers, like modal dialogs.