fix bug where buildings could not output directly on splitters

src/lib/sim/BeltSystem.h

@@ -72,8 +72,9 @@ public:
     // port.direction = direction items flow on that tile
     //
     // tryPutItem: place item onto tile.
-    //   Returns false if the tile is not a belt, or tile full.
-    bool tryPutItem(QPoint tile, Item item);
+    //   Returns false if the tile is not a belt/splitter, or tile full.
+    //   fromDir: travel direction of the item (used for splitter animation).
+    bool tryPutItem(QPoint tile, Item item, Rotation fromDir = Rotation::West);
 
     // tryTakeItem: remove and return the leading item from port.tile.
     //   Returns nullopt if tile is not a belt, direction mismatches, or tile empty.