Provide ol.RendererType structure through ol.renderer.Map

It was previously in ol.Map, but it could not be provided because
of circular dependencies.

src/ol/renderer/maprenderer.js

@@ -1,3 +1,4 @@
+goog.provide('ol.RendererType');
 goog.provide('ol.renderer.Map');
 
 goog.require('goog.Disposable');
@@ -11,6 +12,18 @@ goog.require('ol.style.IconImageCache');
 goog.require('ol.vec.Mat4');
 
 
+/**
+ * Available renderers: `'canvas'`, `'dom'` or `'webgl'`.
+ * @enum {string}
+ * @todo api
+ */
+ol.RendererType = {
+  CANVAS: 'canvas',
+  DOM: 'dom',
+  WEBGL: 'webgl'
+};
+
+
 
 /**
  * @constructor