Don't paint twice

In refactoring the way we generate the SkPicture, I accidentially made us
generate the SkPicture twice. This CL causes us to generate it only once.

packages/flutter/lib/rendering/box.dart

@@ -1678,8 +1678,6 @@ class ViewConstraints {
 }
 
 class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox> {
-  bool get createNewDisplayList => true;
-
   RenderView({
     RenderBox child,
     this.timeForRotation: const Duration(microseconds: 83333)

packages/flutter/lib/rendering/object.dart

@@ -164,7 +164,6 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       return true;
     });
     _nodesNeedingLayout.add(this);
-    _nodesNeedingPaint.add(this);
     scheduler.ensureVisualUpdate();
   }
   static void flushLayout() {
@@ -331,6 +330,9 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       _needsPaint = true;
       _nodesNeedingPaint.add(this);
       scheduler.ensureVisualUpdate();
+    } else if (parent == null) {
+      _needsPaint = true;
+      scheduler.ensureVisualUpdate();
     } else {
       assert(parent != null); // parent always exists on this path because the root node is a RenderView, which sets createNewDisplayList.
       if (parent is RenderObject) {