L.Control.Zoomslider.js : update _onSliderClick() function
L.Control.Zoomslider.js : update _onSliderClick() function

Best use of L.DomEvent.getMousePosition() function.
kartena/Leaflet.zoomslider,kartena/Leaflet.zoomslider,wancharle/Leaflet.zoomslider,wancharle/Leaflet.zoomslider