native: Rename mBuf to mSlot

Remove the union in BufferItem.

Bug: 19769719
Change-Id: I4e496b5aafce0fc5206dc7551a741f9c262c2cd0
diff --git a/libs/gui/BufferItemConsumer.cpp b/libs/gui/BufferItemConsumer.cpp
index 578b8d9..6f4c89d 100644
--- a/libs/gui/BufferItemConsumer.cpp
+++ b/libs/gui/BufferItemConsumer.cpp
@@ -78,7 +78,7 @@
         }
     }
 
-    item->mGraphicBuffer = mSlots[item->mBuf].mGraphicBuffer;
+    item->mGraphicBuffer = mSlots[item->mSlot].mGraphicBuffer;
 
     return OK;
 }
@@ -89,9 +89,9 @@
 
     Mutex::Autolock _l(mMutex);
 
-    err = addReleaseFenceLocked(item.mBuf, item.mGraphicBuffer, releaseFence);
+    err = addReleaseFenceLocked(item.mSlot, item.mGraphicBuffer, releaseFence);
 
-    err = releaseBufferLocked(item.mBuf, item.mGraphicBuffer, EGL_NO_DISPLAY,
+    err = releaseBufferLocked(item.mSlot, item.mGraphicBuffer, EGL_NO_DISPLAY,
             EGL_NO_SYNC_KHR);
     if (err != OK) {
         BI_LOGE("Failed to release buffer: %s (%d)",