Make GUI appear on top of other elements

I was surprised to learn that this didn't default
to z-Index > 0. I think it works in most apps because
it's expected not to call the GUI constructor until after
your other elements have been created? (I didn't look
too deeply)

It's possible you might want to move elements, see
webgpu#330
But in any case, it seemed like a good default.

src/components/SampleLayout.tsx

@@ -90,7 +90,11 @@ const SampleLayout: React.FunctionComponent<
     if (props.gui && process.browser) {
       // eslint-disable-next-line @typescript-eslint/no-var-requires
       const dat = require('dat.gui');
-      return new dat.GUI({ autoPlace: false });
+      const gui = new dat.GUI({ autoPlace: false });
+      // HACK: Make
+      gui.domElement.style.position = 'relative';
+      gui.domElement.style.zIndex = '1000';
+      return gui;
     }
     return undefined;
   }, []);