Add support for a new developer setting: overdraw debugging

Change-Id: I350ba4486577c3289f82c20938f7a35138778727
diff --git a/libs/hwui/Caches.h b/libs/hwui/Caches.h
index 50e9e758..48efd10 100644
--- a/libs/hwui/Caches.h
+++ b/libs/hwui/Caches.h
@@ -242,6 +242,7 @@
     // Misc
     GLint maxTextureSize;
     bool debugLayersUpdates;
+    bool debugOverdraw;
 
     TextureCache textureCache;
     LayerCache layerCache;