webvr: Make canvas focussable

Signed-off-by: Squareys <squareys@googlemail.com>

src/webvr/webvr.html

@@ -10,7 +10,7 @@
   <h1>Magnum WebVR Example</h1>
   <div id="container" class="aspect-2-1">
     <div id="sizer"><div id="expander"><div id="listener">
-      <canvas id="canvas"></canvas>
+      <canvas id="canvas" tabindex="0"></canvas>
       <div id="status">Initialization...</div>
       <div id="status-description"></div>
       <script src="EmscriptenApplication.js"></script>