make write to buf under fill use cx and cz

src/main.lua

@@ -97,7 +97,7 @@ local function fill(ntype, x,y,z, buf, sx,sz)
         for _,v in pairs(newneigh) do
             table.insert(neighbors, v)
         end
-        buf[posasstring(x,y,z)] = ntype
+        buf[posasstring(cx,y,cz)] = ntype
     end
 end
 local function newdesign()