Prevent picturefill from running over the same image multiple times w…

…hen loading async via polling

src/picturefill.js

@@ -146,11 +146,22 @@
         return formattedCandidates;
     };
 
-    w.picturefill = function() {
+    w.picturefill = function(checkEvaluated) {
         // Loop through all images on the page that are `<picture>`
         var pictures = doc.getElementsByTagName("picture");
         for (var i=0, plen = pictures.length; i < plen; i++) {
             var picture = pictures[i];
+
+            // if checkEvaluated is true, we will not re-run on picture elements
+            // that have already been evaluated. This is a useful mode to have
+            // when loading the picturefill async and polling the document
+            // as more of it downloads to fetch images as soon as possible.
+            if (checkEvaluated) {
+                if (picture.hasAttribute('evaluated')) {
+                    continue;
+                };
+                picture.setAttribute('evaluated', true);
+            }
             var matches = [];
 
             // In IE9, <source> elements get removed if they aren't children of
@@ -246,13 +257,15 @@
         var intervalId = setInterval(function(){
             // When the document has finished loading, stop checking for new images
             // https://github.com/ded/domready/blob/master/ready.js#L15
-            w.picturefill();
+            w.picturefill(true);
             if (/^loaded|^i|^c/.test(doc.readyState)) {
                 clearInterval(intervalId);
                 return;
             }
         }, 250);
-        w.addEventListener("resize", w.picturefill, false);
+        w.addEventListener("resize", function() {
+            w.picturefill();
+        }, false);
     };
     runPicturefill();