General purpose canvas renderer

Currently only renders points as circles.  Eventually will have methods for other geometry types and other symbolizer types.  Also provides static methods to render point symbolizers (returning a canvas that can be drawn on another).

src/ol/style/point.js

@@ -6,6 +6,6 @@ goog.require('ol.style.LiteralSymbolizer');
 
 /**
  * @interface
- * @implements {ol.style.LiteralSymbolizer}
+ * @extends {ol.style.LiteralSymbolizer}
  */
 ol.style.LiteralPoint = function() {};