libgui: Add a union to BufferItem for refactoring

Currently, there are two instances of BufferItem: one inside of
IGraphicBufferConsumer, and a standalone one inside of libgui. They
only differ in the name of one of the fields, so this change modifies
the one inside of libgui to have a union of both names so that the
one inside of IGBC can eventually be refactored away.

Change-Id: I64f495105f56cbf5803cea4aa6b072ea29b70cf5
diff --git a/include/gui/BufferItem.h b/include/gui/BufferItem.h
index c7a8bc9..fecdbc1 100644
--- a/include/gui/BufferItem.h
+++ b/include/gui/BufferItem.h
@@ -86,8 +86,13 @@
     // mFrameNumber is the number of the queued frame for this slot.
     uint64_t mFrameNumber;
 
-    // mSlot is the slot index of this buffer (default INVALID_BUFFER_SLOT).
-    int mSlot;
+    union {
+        // mSlot is the slot index of this buffer (default INVALID_BUFFER_SLOT).
+        int mSlot;
+
+        // mBuf is the former name for mSlot
+        int mBuf;
+    };
 
     // mIsDroppable whether this buffer was queued with the
     // property that it can be replaced by a new buffer for the purpose of