Fixing variable interpretation with frames

lib/carto/tree/variable.js

@@ -15,7 +15,7 @@ tree.Variable.prototype = {
         if (this._css) return this._css;
 
         var thisframe = env.frames.filter(function(f) {
-            return f.name == this.name;
+            return f.name == that.name;
         });
         if (thisframe.length) {
             return thisframe[0].value.eval(env);