fixed srcset density parsing

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

@@ -594,7 +594,7 @@ const SingleFileCore = (() => {
 					if (resourceURL && resourceURL != baseURI && DomUtil.testValidPath(resourceURL)) {
 						try {
 							const dataURI = await batchRequest.addURL(new URL(resourceURL, baseURI).href);
-							return dataURI + (srcSetValue.w ? " " + srcSetValue.w + "w" : "");
+							return dataURI + (srcSetValue.w ? " " + srcSetValue.w + "w" : srcSetValue.d ? " " + srcSetValue.d + "x" : "");
 						} catch (e) {
 							// ignored
 						}