Make Tree-Support Deterministic (#15565)

* Make tree support deterministic without giving up its parallelism

* Break equal-distance ties in the tree support MST by coordinates

* test: cover the determinism this PR fixes

The MST unit tests here cover the tie-break, but the drop_nodes rework
has no test.

Adds two cases to the tree support suite. The thread-scheduling one
slices five configs twice each and compares the support point sequence,
which is what the node ordering moves. The MST tie one pins the branch
diameter and line width that carry Prim's equal-distance ties into the
toolpaths.

slice_with_tree_support takes an optional config list so the second case
can add the tree parameters it needs, and the double-slice comparison is
shared rather than written twice.

Both fail on main without this PR. The first passes from 60d1ceb580, the
second from e148865dd6.

---------

Co-authored-by: raistlin7447 <kris.austin@gmail.com>

src/libslic3r/MinimumSpanningTree.cpp

@@ -60,10 +60,15 @@ auto MinimumSpanningTree::prim(std::vector<Point> vertices) const -> AdjacencyGr
         //This search is O(V) right now, which can be made down to O(log(V)). This reduces the overall time complexity from O(V*V) to O(V*log(E)).
         //However that requires an implementation of a heap that supports the decreaseKey operation, which is not in the std library.
         //TODO: Implement this?
+        // Break equal-distance ties on coordinates: the map is keyed by address, so its
+        // iteration order (and therefore the first minimum) would otherwise depend on where
+        // the vertices were allocated.
         using MapValue = std::pair<const Point*, coordf_t>;
         const auto closest = std::min_element(smallest_distance.begin(), smallest_distance.end(),
                                               [](const MapValue& a, const MapValue& b) {
-                                                  return a.second < b.second;
+                                                  if (a.second != b.second)
+                                                      return a.second < b.second;
+                                                  return *a.first < *b.first;
                                               });
 
         //Add this point to the graph and remove it from the candidates.