Using a renderIntent lib function instead of this identifier

A 'this' identifier is quite useless with compressed JavaScript,
and in fact it turned out to fail in advanced mode when trying
to access a feature's renderIntent property with it. The added
renderIntent lib function as a Call expression does the job
well.

src/ol/expr/expression.js

@@ -99,6 +99,7 @@ ol.expr.functions = {
   EXTENT: 'extent',
   FID: 'fid',
   GEOMETRY_TYPE: 'geometryType',
+  RENDER_INTENT: 'renderIntent',
   INTERSECTS: 'intersects',
   CONTAINS: 'contains',
   DWITHIN: 'dwithin',
@@ -252,6 +253,17 @@ ol.expr.lib[ol.expr.functions.GEOMETRY_TYPE] = function(type) {
 };
 
 
+/**
+ * Determine if a feature's renderIntent matches the given one.
+ * @param {string} renderIntent Render intent.
+ * @return {boolean} The feature's renderIntent matches the given one.
+ * @this {ol.Feature}
+ */
+ol.expr.lib[ol.expr.functions.RENDER_INTENT] = function(renderIntent) {
+  return this.renderIntent == renderIntent;
+};
+
+
 ol.expr.lib[ol.expr.functions.INTERSECTS] = function(geom, opt_projection,
     opt_attribute) {
   throw new Error('Spatial function not implemented: ' +