KaTex PoC: Add snippet of html2canvas

With this, maybe we don't even need to have a screenshot ability, and
can just pass the image encoded as a message.

lib/main_webview.dart

@@ -269,3 +269,12 @@ class _MyHomePageState extends State<MyHomePage> {
     );
   }
 }
+
+/*
+
+html2canvas(document.body).then(function(canvas) {
+    var img = canvas.toDataURL("image/png");
+    console.log(img);
+});
+
+*/