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/Blocks.java

@@ -103,7 +103,7 @@ public class Blocks{
     overflowGate, underflowGate, massDriver,
 
     //transport - alternate
-    duct, armoredDuct, ductRouter, overflowDuct, ductBridge, ductUnloader,
+    duct, armoredDuct, ductRouter, overflowDuct, underflowDuct, ductBridge, ductUnloader,
     surgeConveyor, surgeRouter,
 
     unitCargoLoader, unitCargoUnloadPoint,
@@ -1991,6 +1991,15 @@ public class Blocks{
             researchCostMultiplier = 1.5f;
         }};
 
+        underflowDuct = new OverflowDuct("underflow-duct"){{
+            requirements(Category.distribution, with(Items.graphite, 8, Items.beryllium, 8));
+            health = 90;
+            speed = 4f;
+            solid = false;
+            researchCostMultiplier = 1.5f;
+            invert = true;
+        }};
+
         ductBridge = new DuctBridge("duct-bridge"){{
             requirements(Category.distribution, with(Items.beryllium, 20));
             health = 90;
@@ -5316,4 +5325,4 @@ public class Blocks{
 
         //endregion
     }
-}
+}