Add built-in sphere mesh.

PiperOrigin-RevId: 791162850
Change-Id: I573a414727e79f1a495e0f2282b2faff48ab3443

src/user/user_objects.h

@@ -1052,6 +1052,7 @@ class mjCMesh: public mjCMesh_, private mjsMesh {
 
   // make a mesh of a predefined shape
   void MakeHemisphere(int res, bool make_faces, bool make_cap);
+  void MakeSphere(int subdiv, bool make_faces);
   void MakeWedge(int resolution[2], double fov[2], double gamma);
   void MakeRect(int resolution[2]);
   void MakeCone(int nedge, double radius);