Support future syntax of agg-stack-blur

lib/carto/functions.js

@@ -143,6 +143,10 @@ for (var i = 0; i < image_filter_functors.length; i++) {
     })(f);
 }
 
+tree.functions['agg-stack-blur'] = function(x, y) {
+    return new tree.ImageFilter('agg-stack-blur', [x, y]);
+}
+
 
 function hsla(hsla) {
     return tree.functions.hsla(hsla.h, hsla.s, hsla.l, hsla.a);

lib/carto/tree/imagefilter.js

@@ -12,7 +12,7 @@ tree.ImageFilter.prototype = {
 
     toString: function() {
         if (this.args) {
-            // TODO
+            return this.filter + ':' + this.args.join(',');
         } else {
             return this.filter;
         }

test/rendering/image_filters.mss

@@ -2,7 +2,7 @@
   polygon-fill: #FFF;
   line-color:#F00;
   line-width: 0.5;
-  image-filters: blur(), sharpen();
+  image-filters: blur() sharpen() agg-stack-blur(2, 2);
   composite-operation: src-in;
   line-dasharray: 2, 3;
 }