EPUB: support srcset

Closes #98

epub.js

@@ -836,7 +836,6 @@ class Loader {
                 }
             }
             // replace hrefs (excluding anchors)
-            // TODO: srcset?
             const replace = async (el, attr) => el.setAttribute(attr,
                 await this.loadHref(el.getAttribute(attr), href, parents))
             for (const el of doc.querySelectorAll('link[href]')) await replace(el, 'href')
@@ -846,6 +845,11 @@ class Loader {
             for (const el of doc.querySelectorAll('[*|href]:not([href])'))
                 el.setAttributeNS(NS.XLINK, 'href', await this.loadHref(
                     el.getAttributeNS(NS.XLINK, 'href'), href, parents))
+            for (const el of doc.querySelectorAll('[srcset]'))
+                el.setAttribute('srcset', await replaceSeries(el.getAttribute('srcset'),
+                    /(\s*)(.+?)\s*((?:\s[\d.]+[wx])+\s*(?:,|$)|,\s+|$)/g,
+                (_, p1, p2, p3) => this.loadHref(p2, href, parents)
+                    .then(p2 => `${p1}${p2}${p3}`)))
             // replace inline styles
             for (const el of doc.querySelectorAll('style'))
                 if (el.textContent) el.textContent =