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/include/gui/BufferItem.h b/include/gui/BufferItem.h
index 370f5d5..a515f39 100644
--- a/include/gui/BufferItem.h
+++ b/include/gui/BufferItem.h
@@ -125,6 +125,10 @@
     // Indicates that this buffer was queued by the producer. When in single
     // buffer mode acquire() can return a BufferItem that wasn't in the queue.
     bool mQueuedBuffer;
+
+    // Indicates that this BufferItem contains a stale buffer which has already
+    // been released by the BufferQueue.
+    bool mIsStale;
 };
 
 } // namespace android