Fix levels node regression in tiled compositor

Caused by previous change for full-frame compositor.

The determine_canvas() is still used by the tiled compositor,
so restore it to the state which makes both compositors to work.

Pull Request: https://projects.blender.org/blender/blender/pulls/118254

source/blender/compositor/operations/COM_CalculateMeanOperation.cc

@@ -157,6 +157,7 @@ void CalculateMeanOperation::get_area_of_interest(int input_idx,
 
 void CalculateMeanOperation::determine_canvas(const rcti &preferred_area, rcti &r_area)
 {
+  ConstantOperation::determine_canvas(preferred_area, r_area);
   r_area = preferred_area;
 }