Skip non-img nodes when using options.elements

Prevent processing when `options.elements` contains non-`img` nodes.

Author: Christophe de Jonge

dist/picturefill.js

@@ -503,6 +503,11 @@ window.matchMedia || (window.matchMedia = function() {
 			firstMatch = undefined;
 			candidates = undefined;
 
+			// immediately skip non-`img` nodes
+			if ( element.nodeName.toUpperCase() !== "IMG" ) {
+				continue;
+			}
+
 			// expando for caching data on the img
 			if ( !element[ pf.ns ] ) {
 				element[ pf.ns ] = {};

dist/picturefill.min.js

@@ -454,6 +454,11 @@
 			firstMatch = undefined;
 			candidates = undefined;
 
+			// immediately skip non-`img` nodes
+			if ( element.nodeName.toUpperCase() !== "IMG" ) {
+				continue;
+			}
+
 			// expando for caching data on the img
 			if ( !element[ pf.ns ] ) {
 				element[ pf.ns ] = {};