function startRender (sceneEl) {
            if (sceneEl.renderStarted) { return; }

            sceneEl.resize();

            // Kick off render loop.
            if (sceneEl.renderer) {
              if (window.performance) {
                window.performance.mark('render-started');
              }
              sceneEl.clock = new THREE.Clock();
              sceneEl.render();
              sceneEl.renderStarted = true;
              sceneEl.emit('renderstart');
            }
          }
Handler attached to elements to help scene know when to kick off.
Scene waits for all entities to load.