Cleanup: Better use references than copy by value for PointerRNA now.

While copying this used to be very cheap, this will soon be less
trivial.

Note that there are many more places where this type of cleanup will be
needed.

source/blender/python/intern/bpy_rna_anim.cc

@@ -375,7 +375,7 @@ PyObject *pyrna_struct_keyframe_insert(BPy_StructRNA *self, PyObject *args, PyOb
      * not have any effect.
      */
 
-    PointerRNA ptr = self->ptr;
+    PointerRNA &ptr = self->ptr;
     PropertyRNA *prop = nullptr;
     const char *prop_name;