graphviz: allow overriding the engine

This can be done like so:

  {{< graphviz engine="neato" >}}

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

layouts/_default/baseof.html

@@ -204,7 +204,8 @@ <h1 data-pagefind-meta="title">About{{ if (isset .Params "subtitle") }} - {{ .Pa
     let vizInstance
     [...document.querySelectorAll("pre[class=graphviz]")].forEach(async (x) => {
       if (!vizInstance) vizInstance = await Viz.instance()
-      const svg = vizInstance.renderSVGElement(x.innerText)
+      const engine = x.getAttribute("engine") || undefined
+      const svg = vizInstance.renderSVGElement(x.innerText, { engine })
       x.parentNode.insertBefore(svg, x);
       x.style.display = 'none'
     });

layouts/shortcodes/graphviz.html

@@ -1,4 +1,4 @@
-<pre class="graphviz">
+<pre class="graphviz"{{ if (ne "" (.Get "engine")) }} engine="{{(.Get "engine")}}"{{ end }}>
   {{ .Inner | htmlEscape | safeHTML }}
 </pre>
 {{ .Page.Store.Set "hasGraphviz" true }}