Underflow duct (#6957)

* Underflow Gate

I'm not sure why ClassMap decided to import OverflowDuct instead of using the full path when being generated.

* Better top sprite

Also fix base asymmetry

Co-authored-by: Anuken <arnukren@gmail.com>

core/src/mindustry/content/ErekirTechTree.java

@@ -87,6 +87,7 @@ public class ErekirTechTree{
                     });
 
                     node(overflowDuct, Seq.with(new OnSector(two)), () -> {
+                        node(underflowDuct);
                         node(reinforcedContainer, () -> {
                             node(ductUnloader, () -> {