custom models: option to copy mod/eye draws from original model (#3800)

Two new flags were added to the Blender plugin to allow reusing the mod
and/or eye draws of the original model that is being replaced. Works
pretty well for eyes, but the blerc draws can cause some Z-fighting with
the non-moving parts of the model.

Also a small refactor to the merc replacement code to de-duplicate some
code by moving stuff to `gltf_util.cpp`.

decompiler/IR2/FormExpressionAnalysis.cpp

@@ -2603,9 +2603,6 @@ void SetVarElement::push_to_stack(const Env& env, FormPool& pool, FormStack& sta
     ASSERT(x->parent_form == m_src);
   }
 
-  if (auto test0 = m_src->to_string(env) == "(* 0.00024414062 (-> arg0 y))") {
-    printf("");
-  }
   if (m_src->is_single_element()) {
     auto src_as_se = dynamic_cast<SimpleExpressionElement*>(m_src->back());
     if (src_as_se) {