Scrolling performance improvements

- Avoid overly aggressive backing store purge on unmount
- Avoid possible 2 draw calls in a single frame

Author: Michael Johnston

lib/LayerMixin.js

@@ -81,11 +81,6 @@ var LayerMixin = {
   },
 
   unmountComponent: function() {
-    // Purge backing stores on unmount.
-    var layer = this.node;
-    if (layer.backingStoreId) {
-      DrawingUtils.invalidateBackingStore(layer.backingStoreId);
-    }
     this.destroyEventListeners();
   }
 

lib/Surface.js

@@ -145,8 +145,8 @@ var Surface = React.createClass({
     // Execute pending draw that may have been scheduled during previous frame
     this._frameReady = true;
     if (this._pendingTick) {
-      this.tick();
       this._pendingTick = false;
+      this.batchedTick();
     }
   },