pgraph: Mark `RenderState::unref()` as final

This will allow the compiler to devirtualize these calls

panda/src/pgraph/renderState.h

@@ -112,7 +112,7 @@ PUBLISHED:
 
   INLINE CPT(RenderState) get_unique() const;
 
-  virtual bool unref() const;
+  virtual bool unref() const final;
 
   INLINE void cache_ref() const;
   INLINE bool cache_unref() const;