Copy only referenced plugins.

Fixes #2111.

PiperOrigin-RevId: 681486400
Change-Id: I384ac63fa76b27d6e84d91fa857127c42ff52e34

src/user/user_objects.h

@@ -824,6 +824,8 @@ class mjCMesh_ : public mjCBase {
 };
 
 class mjCMesh: public mjCMesh_, private mjsMesh {
+  friend class mjCModel;
+
  public:
   mjCMesh(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCMesh(const mjCMesh& other);