tpl/tplimpl: Extend page image lookup to include global resources

Fall back to global resources via resources.Get when page resources
don't match for named images in the images front matter parameter.
This aligns get-page-images.html with the existing behavior in
render-image.html, render-link.html, and figure.html.

Fixes #14062

tpl/tplimpl/embedded/templates/_partials/_funcs/get-page-images.html

@@ -23,9 +23,8 @@
   {{- $img := . }}
   {{- $url := urls.Parse $img }}
   {{- if eq $url.Scheme "" }}
-    {{/* Internal image. */}}
-    {{- with $resources.GetMatch $img -}}
-      {{/* Image resource. */}}
+    {{/* Internal image: check page resources first, then global resources. */}}
+    {{- with or ($resources.GetMatch $img) (resources.Get $img) -}}
       {{- $imgs = $imgs | append (dict
         "Image" .
         "RelPermalink" .RelPermalink