Giving vector layers the style they deserve

Now vector layers can have a style. ol.Style instances have an
apply method to get the symbolizer literals for a feature. If the
layer does not have a style defined, there is also a static
applyDefaultStyle function on ol.Style to get the default
symbolizer literals for a feature. The vector layer also got a
groupFeaturesBySymbolizerLiteral method, which returns an array
with features grouped by symbolizer, as needed by the canvas
renderer.

src/ol/style/symbolizer.js

@@ -11,6 +11,14 @@ goog.require('ol.Feature');
 ol.style.SymbolizerLiteral = function() {};
 
 
+/**
+ * @param {ol.style.SymbolizerLiteral} symbolizerLiteral Symbolizer literal to
+ *     compare to.
+ * @return {boolean} Is the passed symbolizer literal equal to this instance?
+ */
+ol.style.SymbolizerLiteral.prototype.equals = goog.abstractMethod;
+
+
 
 /**
  * @constructor