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/Support/TreeSupport3D.cpp

@@ -2382,13 +2382,10 @@ static void merge_influence_areas(
     size_t num_buckets_initial;
     {
         // How many buckets per first merge iteration?
-        const size_t num_threads     = tbb::this_task_arena::max_concurrency();
-        // 4 buckets per thread if possible,
-        const size_t num_buckets_min = (input_size + 2) / 4;
-        // 2 buckets per thread otherwise.
-        const size_t num_buckets_max = input_size / 2;
-        num_buckets_initial          = num_buckets_min >= num_threads ? num_buckets_min : num_buckets_max;
-        const size_t bucket_size     = num_buckets_min >= num_threads ? 4 : 2;
+        // Fixed at 4: merging is not associative, so sizing buckets off max_concurrency() made
+        // results depend on the core count of the slicing machine.
+        const size_t bucket_size     = 4;
+        num_buckets_initial          = (input_size + 2) / 4;
         // Fill in the buckets.
         SupportElementMerging *it = influence_areas.data();
         // Reserve one more bucket to keep a single influence area which will not be merged in the first iteration.