fixed CSS images grouping

lib/single-file/single-file-core.js

@@ -1088,7 +1088,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 				}
 			}
 
-			function getURLFunctions(cssRules, urlFunctions = { style: new Set(), other: new Set() }) {
+			function getURLFunctions(cssRules, urlFunctions = { style: [], other: [] }) {
 				Array.from(cssRules).forEach(cssRule => {
 					if (cssRule.type == CSSRule.MEDIA_RULE) {
 						getURLFunctions(cssRule.cssRules, urlFunctions);
@@ -1108,7 +1108,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 					const resourceURL = DomUtil.normalizeURL(originalResourceURL);
 					if (!DomUtil.testIgnoredPath(resourceURL)) {
 						if (DomUtil.testValidURL(resourceURL, baseURI, options.url) && cssText.includes(urlFunction)) {
-							urlFunctions.add(urlFunction);
+							urlFunctions.push(urlFunction);
 						}
 					}
 				});