Unsnap events don't get fired reliably

[sidney-pauly]

If a marker gets snapped to another marker before it was unsnapped from from the previous marker, no unsnap event is fired. Here is a working proposal to fix the issue by first firing the unsnap event before doing a new snap:

L.Snap.updateSnap = function (marker, layer, latlng) {
    if (! marker.hasOwnProperty('_latlng')) {
        return;
    }

    if(marker.snap != layer){
        if (marker.snap) {
            if (marker._icon) {
                L.DomUtil.removeClass(marker._icon, 'marker-snapped');
            }
            marker.fire('unsnap', {layer: marker.snap});
        }
        
        delete marker.snap;
    }

    if (layer && latlng) {
        // don't call setLatLng so that we don't fire an unnecessary 'move' event
        marker._latlng = L.latLng(latlng);
        marker.update();
        if (marker.snap != layer) {
            marker.snap = layer;
            if (marker._icon) {
                L.DomUtil.addClass(marker._icon, 'marker-snapped');
            }
            marker.fire('snap', {layer:layer, latlng: latlng});
        }
    }
    
};