Zoomslider cannot be added to a map with no target

The zoom slider control's initSlider_ function requires that the control's element is inserted in the document. So if initSlider_ is called before the map have a target then the slider isn't correctly initialized. This commit fixes that by defering the slider initialization until the first handleMapPostrender call.

test/spec/ol/control/zoomslidercontrol.test.js

@@ -68,6 +68,7 @@ describe('ol.control.ZoomSlider', function() {
       control.element.style.width = '1000px';
       control.element.style.height = '10px';
       control.setMap(map);
+      control.initSlider_();
 
       var horizontal = ol.control.ZoomSlider.direction.HORIZONTAL;
       expect(control.direction_).to.be(horizontal);