Use std::stable_sort in model compiler.

PiperOrigin-RevId: 538791034
Change-Id: I6c8eceffeb765a8a9801fee996d6ebf2964d8453

src/user/user_mesh.cc

@@ -318,7 +318,7 @@ void mjCMesh::Compile(int vfs_provider) {
 
   // check for inconsistent face orientations
   if (!useredge.empty()) {
-    std::sort(useredge.begin(), useredge.end());
+    std::stable_sort(useredge.begin(), useredge.end());
     auto iterator = std::adjacent_find(useredge.begin(), useredge.end());
     if (iterator != useredge.end()) {
       invalidorientation.first = iterator->first+1;

src/user/user_model.cc

@@ -2619,8 +2619,8 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, int vfs_provider) {
   }
 
   // sort pair, exclude in increasing signature order; reassign ids
-  sort(pairs.begin(), pairs.end(), comparePair);
-  sort(excludes.begin(), excludes.end(), compareBodyPair);
+  std::stable_sort(pairs.begin(), pairs.end(), comparePair);
+  std::stable_sort(excludes.begin(), excludes.end(), compareBodyPair);
   reassignid(pairs);
   reassignid(excludes);