Add support for multiple textures in one material (mjModel).

PiperOrigin-RevId: 648732764
Change-Id: I29b60dd62a069789a32017ff0d55e36b57e0052b

src/user/user_model.cc

@@ -2481,7 +2481,10 @@ void mjCModel::CopyObjects(mjModel* m) {
     mjCMaterial* pmat = materials_[i];
 
     // set fields
-    m->mat_texid[i] = pmat->texid;
+    m->mat_texid[mjNTEXMAT*i] = pmat->texid;
+    for (int j=1; j<mjNTEXMAT; j++) {
+      m->mat_texid[mjNTEXMAT*i+j] = -1;
+    }
     m->mat_texuniform[i] = pmat->texuniform;
     mjuu_copyvec(m->mat_texrepeat+2*i, pmat->texrepeat, 2);
     m->mat_emission[i] = pmat->emission;