perf: switch source and target for dijkstra

When exploring from the root, the tendency will be to
explore around all the zero weighted previously traced paths
whereas if you start from the target, you'll locally explore
(which we had to do anyway) and the soon as you find a zero
weighted path you accelerate towards the root.

kimimaro/trace.py

@@ -205,7 +205,11 @@ def compute_paths(
       target = kimimaro.skeletontricks.find_target(labels, DAF)
 
     if fix_branching:
-      path = dijkstra3d.dijkstra(parents, root, target, bidirectional=soma_mode)
+      # faster to trace from target to root than root to target
+      # because that way local exploration finds any zero weighted path
+      # and finishes vs exploring from the neighborhood of the entire zero
+      # weighted path
+      path = dijkstra3d.dijkstra(parents, target, root, bidirectional=soma_mode)
     else:
       path = dijkstra3d.path_from_parents(parents, target)