Re-enable top level clear

Author: Avaer Kazmer

src/index.js

@@ -453,7 +453,7 @@ const _computeDerivedGamepadsData = () => {
   }
 };
 const _tickAnimationFrame = win => {
-  // win.clear();
+  win.clear();
   return win.runAsync({
     method: 'tickAnimationFrame',
     layered: true,

src/xr-scene.js

@@ -92,13 +92,13 @@ class XRScene extends HTMLElement {
           }
           return win.ctx;
         };
-        /* win.clear = () => {
+        win.clear = () => {
           if (win.ctx) {
             win.ctx.binding = null;
             win.ctx.clearColor(0, 0, 0, 1);
             win.ctx.clear(win.ctx.COLOR_BUFFER_BIT|win.ctx.STENCIL_BUFFER_BIT|win.ctx.DEPTH_BUFFER_BIT);
           }
-        }; */
+        };
         win.destroy = (destroy => function() {
           if (win.canvas) {
             this.shadow.removeChild(win.canvas);