libtrellis: Fix IOLOGIC Bels z

Signed-off-by: David Shah <dave@ds0.me>

libtrellis/src/Bels.cpp

diff --git a/libtrellis/src/Bels.cpp b/libtrellis/src/Bels.cpp
index 135ddae..2a8154e 100644
--- a/libtrellis/src/Bels.cpp
+++ b/libtrellis/src/Bels.cpp
@@ -379,7 +379,7 @@
     bel.type = graph.ident(ss + "IOLOGIC");
     bel.loc.x = x;
     bel.loc.y = y;
-    bel.z = z + 4;
+    bel.z = z + (s ? 2 : 4);
 
     auto add_input = [&](const std::string &pin, bool j = true) {
         graph.add_bel_input(bel, graph.ident(pin), x, y, graph.ident(fmt((j ? "J" : "") << pin << l << "_" << ss << "IOLOGIC")));