BQ: Improved buffer/slot tracking

- Explicitly track active buffers and unused slots on top of the
  already existing tracking for free slots and free buffers.

Change-Id: Ife2678678e96f0eb0b3fb21571058378134bd868
diff --git a/libs/gui/BufferItem.cpp b/libs/gui/BufferItem.cpp
index de8ff70..036ef1e 100644
--- a/libs/gui/BufferItem.cpp
+++ b/libs/gui/BufferItem.cpp
@@ -39,7 +39,8 @@
     mTransformToDisplayInverse(false),
     mSurfaceDamage(),
     mSingleBufferMode(false),
-    mQueuedBuffer(true) {
+    mQueuedBuffer(true),
+    mIsStale(false) {
 }
 
 BufferItem::~BufferItem() {}