Create z reordering boundaries around dispatchDraw

bug:16012254

This means rendernodes with a Z will no longer be drawn at the end of
their parent's DisplayList, but at the end of the associated reorder
region (DisplayListData::Chunk).

Change-Id: Ia033fee9d9a4db567b2a8d5e90fc57a4d0a64544
diff --git a/libs/hwui/DisplayList.cpp b/libs/hwui/DisplayList.cpp
index 6461ee7..d8932ce 100644
--- a/libs/hwui/DisplayList.cpp
+++ b/libs/hwui/DisplayList.cpp
@@ -89,11 +89,9 @@
     layers.clear();
 }
 
-void DisplayListData::addChild(DrawRenderNodeOp* op) {
-    LOG_ALWAYS_FATAL_IF(!op->renderNode(), "DrawRenderNodeOp with no render node!");
-
-    mChildren.push(op);
+size_t DisplayListData::addChild(DrawRenderNodeOp* op) {
     mReferenceHolders.push(op->renderNode());
+    return mChildren.add(op);
 }
 
 }; // namespace uirenderer