Overlay visible before updating render position

Make the overlay visible before updating the render position
so that the `offsetWidth` and `offsetHeight` can be 
considered in evaluating the offset and positioning of the 
element

See openlayers/openlayers#6566

src/ol/overlay.js

@@ -464,6 +464,8 @@ ol.Overlay.prototype.updateRenderedPosition = function(pixel, mapSize) {
 
   var positioning = this.getPositioning();
 
+  this.setVisible(true);
+  
   var offsetX = offset[0];
   var offsetY = offset[1];
   if (positioning == ol.OverlayPositioning.BOTTOM_RIGHT ||
@@ -514,8 +516,6 @@ ol.Overlay.prototype.updateRenderedPosition = function(pixel, mapSize) {
       this.rendered_.top_ = style.top = top;
     }
   }
-
-  this.setVisible(true);
 };