Fix janky pose

It's introduced through ag/1980993, where render_buffer_index_ was not
managed after the refactor.

Bug: 36194745
Test: Build and flash, pose is no longer janky.
Change-Id: I6ef86625dbc1c1afd2725614062f5e40e1b7f60a
diff --git a/libs/vr/libvrflinger/display_surface.h b/libs/vr/libvrflinger/display_surface.h
index feb173e..d31a3a9 100644
--- a/libs/vr/libvrflinger/display_surface.h
+++ b/libs/vr/libvrflinger/display_surface.h
@@ -60,10 +60,7 @@
     }
   }
 
-  uint32_t GetRenderBufferIndex(int buffer_id) {
-    return buffer_id_to_index_[buffer_id];
-  }
-
+  uint32_t GetRenderBufferIndex(int buffer_id);
   bool IsBufferAvailable();
   bool IsBufferPosted();
   AcquiredBuffer AcquireCurrentBuffer();
@@ -172,6 +169,8 @@
   float manager_blur_;
   int layer_order_;
 
+  // The monotonically increasing index for allocated buffers in this surface.
+  uint32_t allocated_buffer_index_;
   // Maps from the buffer id to the corresponding allocated buffer index.
   std::unordered_map<int, uint32_t> buffer_id_to_index_;
 };