Allow source to be set after layer construction

src/ol/renderer/dom/domimagelayerrenderer.js

@@ -69,6 +69,15 @@ ol.renderer.dom.ImageLayer.prototype.forEachFeatureAtPixel =
 };
 
 
+/**
+ * @inheritDoc
+ */
+ol.renderer.dom.ImageLayer.prototype.clearFrame = function() {
+  goog.dom.removeChildren(this.target);
+  this.image_ = null;
+};
+
+
 /**
  * @inheritDoc
  */

src/ol/renderer/dom/domlayerrenderer.js

@@ -27,6 +27,12 @@ ol.renderer.dom.Layer = function(mapRenderer, layer, target) {
 goog.inherits(ol.renderer.dom.Layer, ol.renderer.Layer);
 
 
+/**
+ * Clear rendered elements.
+ */
+ol.renderer.dom.Layer.prototype.clearFrame = goog.nullFunction;
+
+
 /**
  * @param {olx.FrameState} frameState Frame state.
  * @param {ol.layer.LayerState} layerState Layer state.

src/ol/renderer/dom/dommaprenderer.js

@@ -244,6 +244,8 @@ ol.renderer.dom.Map.prototype.renderFrame = function(frameState) {
       if (layerRenderer.prepareFrame(frameState, layerState)) {
         layerRenderer.composeFrame(frameState, layerState);
       }
+    } else {
+      layerRenderer.clearFrame();
     }
   }
 

src/ol/renderer/dom/domtilelayerrenderer.js

@@ -74,6 +74,15 @@ ol.renderer.dom.TileLayer = function(mapRenderer, tileLayer) {
 goog.inherits(ol.renderer.dom.TileLayer, ol.renderer.dom.Layer);
 
 
+/**
+ * @inheritDoc
+ */
+ol.renderer.dom.TileLayer.prototype.clearFrame = function() {
+  goog.dom.removeChildren(this.target);
+  this.renderedRevision_ = 0;
+};
+
+
 /**
  * @inheritDoc
  */