added utility function eval to get the real value for a property

lib/carto/renderer_js.js

@@ -192,6 +192,11 @@ CartoCSS.Layer.prototype = {
     return this.fullName().split('::')[1];
   },
 
+  eval: function(prop) {
+    var p = this.shader[prop];
+    if (!p) return;
+    return p({}, { zoom: 0, 'frame-offset': 0 });
+  },
 
   /*
    * `target`: style, 'svg', 'canvas-2d'...