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/line.js

@@ -37,6 +37,16 @@ ol.style.LineLiteral = function(config) {
 goog.inherits(ol.style.LineLiteral, ol.style.SymbolizerLiteral);
 
 
+/**
+ * @inheritDoc
+ */
+ol.style.LineLiteral.prototype.equals = function(lineLiteral) {
+  return this.strokeStyle == lineLiteral.strokeStyle &&
+      this.strokeWidth == lineLiteral.strokeWidth &&
+      this.opacity == lineLiteral.opacity;
+};
+
+
 /**
  * @typedef {{strokeStyle: (string|ol.Expression),
  *            strokeWidth: (number|ol.Expression),