MultiThreaded rendering of different renderNodes

This is adding the renderer side infrastructure to allow
rendering multiple render nodes with different threads.
This is a pre-step for decoupling a non client decor
resize reder from a content resize render.

Multiple render nodes can be added to be drawn, and to
prevent overdrawing, a content bounds area can be set

Bug: 22527834

Change-Id: Ie7271e20895bf38957e5a84aeefc883e282039ad
diff --git a/libs/hwui/TreeInfo.h b/libs/hwui/TreeInfo.h
index ed853f7..98e6146 100644
--- a/libs/hwui/TreeInfo.h
+++ b/libs/hwui/TreeInfo.h
@@ -77,7 +77,7 @@
         , canvasContext(clone.canvasContext)
     {}
 
-    const TraversalMode mode;
+    TraversalMode mode;
     // TODO: Remove this? Currently this is used to signal to stop preparing
     // textures if we run out of cache space.
     bool prepareTextures;