Fix #127051: Compositing using multiple Levels nodes freezes Blender

Multiple levels nodes in a row causes blender to freeze when combined with other constant operations. Also, current results of standard deviation are wrong.

The problem was that constant folding optimized constant operations away, which caused a cyclic node graph, and therefore the freeze during node graph evaluation. Preventing such optimizations solves the problem.

Pull Request: https://projects.blender.org/blender/blender/pulls/127316

source/blender/compositor/operations/COM_CalculateMeanOperation.cc

@@ -16,7 +16,9 @@ CalculateMeanOperation::CalculateMeanOperation()
   this->add_output_socket(DataType::Value);
   is_calculated_ = false;
   setting_ = 1;
-  flags_.is_constant_operation = true;
+
+  /* Prevent optimization by constant folder. */
+  flags_.is_constant_operation = false;
 
   needs_canvas_to_get_constant_ = true;
 }