Make ol.renderer.Map behave like a null renderer, fixes #82

With this patch, creating a new ol.renderer.Map will return a map
renderer that does nothing.  Previously, it could cause assertion
failures.

src/ol/renderer/dom/dommaprenderer.js

@@ -228,7 +228,15 @@ ol.renderer.dom.Map.prototype.render = function() {
   this.renderedRotation_ = mapRotation;
   this.renderedSize_ = mapSize;
 
-  return goog.base(this, 'render');
+  var animate = false;
+  this.forEachReadyVisibleLayer(function(layer, layerRenderer) {
+    if (layerRenderer.render()) {
+      animate = true;
+    }
+  });
+
+  return animate;
+
 };
 
 

src/ol/renderer/maprenderer.js

@@ -115,7 +115,9 @@ ol.renderer.Map.prototype.canRotate = goog.functions.FALSE;
  * @protected
  * @return {ol.renderer.Layer} layerRenderer Layer renderer.
  */
-ol.renderer.Map.prototype.createLayerRenderer = goog.abstractMethod;
+ol.renderer.Map.prototype.createLayerRenderer = function(layer) {
+  return null;
+};
 
 
 /**
@@ -310,15 +312,7 @@ ol.renderer.Map.prototype.removeLayerRenderer = function(layer) {
 /**
  * @return {boolean} Animating.
  */
-ol.renderer.Map.prototype.render = function() {
+ol.renderer.Map.prototype.render = goog.functions.FALSE;
-  var animate = false;
-  this.forEachReadyVisibleLayer(function(layer, layerRenderer) {
-    if (layerRenderer.render()) {
-      animate = true;
-    }
-  });
-  return animate;
-};
 
 
 /**

src/ol/renderer/webgl/webglmaprenderer.js

@@ -562,7 +562,12 @@ ol.renderer.webgl.Map.prototype.render = function() {
     this.canvasSize_ = size;
   }
 
-  var animate = goog.base(this, 'render');
+  var animate = false;
+  this.forEachReadyVisibleLayer(function(layer, layerRenderer) {
+    if (layerRenderer.render()) {
+      animate = true;
+    }
+  });
 
   var gl = this.getGL();