Remove box, cylinder and ellipsoid composite types.

Also, fix a bug with Flex textures that was causing an incorrect allocation of the textures in mjModel. Fixes #2013.

PiperOrigin-RevId: 692510858
Change-Id: If781716216974e085244da27bc3aa57d91c3458c

src/user/user_model.cc

@@ -1613,6 +1613,7 @@ void mjCModel::SetSizes() {
     nflexelemedge += flexes_[i]->nelem * mjCFlex::kNumEdges[flexes_[i]->dim - 1];
     nflexshelldata += (int)flexes_[i]->shell.size();
     nflexevpair += (int)flexes_[i]->evpair.size()/2;
+    nflextexcoord += (flexes_[i]->HasTexcoord() ? flexes_[i]->get_texcoord().size()/2 : 0);
   }
 
   // mesh counts