remove assumption that world starts at Y=0

platforms/region/src/main/java/com/dfsek/terra/platform/DirectWorld.java

@@ -92,6 +92,11 @@ public class DirectWorld implements World {
         return null;
     }
 
+    @Override
+    public int getMinHeight() {
+        return 0;
+    }
+
     @Override
     public Object getHandle() {
         return generator;