perf: lazy-load lightbox duplicate

The implementation of a lightbox requires that the image in question is
rendered twice, once in the actual content and a second time as an
off-screen image. Defer loading of that off-screen duplicate until it is
needed.

Signed-off-by: Luca Zeuch <[email protected]>

Author: l-zeuch

layouts/_default/_markup/render-image.html

@@ -124,7 +124,7 @@
 {{- if $lightbox -}}
 </a>
 <a href="javascript:history.back();" class="lightbox" id="{{ print $id "-lightbox" }}">
-  <img
+  <img loading="lazy"
   {{- range $k, $v := $attrs }}
     {{- if or $v (eq $k "alt") }}
       {{- printf " %s=%q" $k $v | safeHTMLAttr }}