fix direct-image-filters output and add test for image-filters

lib/carto/tree/style.js

@@ -39,7 +39,7 @@ tree.StyleXML = function(name, attachment, definitions, env) {
     }
 
     if (direct_image_filters.length) {
-        attrs_xml += ' direct-image-filters="' + _.chain(image_filters)
+        attrs_xml += ' direct-image-filters="' + _.chain(direct_image_filters)
             // prevent identical filters from being duplicated in the style
             .uniq(function(i) { return i.id; }).map(function(f) {
             return f.ev(env).toXML(env, true, ',', 'direct-image-filter');

test/rendering-mss/image-filters.mss

@@ -0,0 +1,4 @@
+#layer {
+	image-filters:invert();
+	direct-image-filters:invert();
+}

test/rendering-mss/image-filters.xml

@@ -0,0 +1,2 @@
+<Style name="style" filter-mode="first" image-filters="invert" direct-image-filters="invert">
+</Style>