Prevents plastanium conveyor unload exploit (#4098)

* Prevent unloaders from unloading plast loading dock

* Use overridable boolean method unloadable instead

* unloadable -> canUnload

core/src/mindustry/entities/comp/BuildingComp.java

@@ -384,6 +384,10 @@ abstract class BuildingComp implements Posc, Teamc, Healthc, Buildingc, Timerc,
     //endregion
     //region handler methods
 
+    public boolean canUnload(){
+        return block.unloadable;
+    }
+
     /** Called when an unloader takes an item. */
     public void itemTaken(Item item){
 

core/src/mindustry/world/blocks/distribution/StackConveyor.java

@@ -179,6 +179,11 @@ public class StackConveyor extends Block implements Autotiler{
             }
         }
 
+        @Override
+        public boolean canUnload(){
+            return state != stateLoad;
+        }
+
         @Override
         public float efficiency(){
             return 1f;

core/src/mindustry/world/blocks/storage/Unloader.java

@@ -61,7 +61,7 @@ public class Unloader extends Block{
                     int pos = (offset + i) % proximity.size;
                     var other = proximity.get(pos);
 
-                    if(other.interactable(team) && other.block.unloadable && other.block.hasItems
+                    if(other.interactable(team) && other.block.unloadable && other.canUnload() && other.block.hasItems
                     && ((sortItem == null && other.items.total() > 0) || (sortItem != null && other.items.has(sortItem)))){
                         //make sure the item can't be dumped back into this block
                         dumpingTo = other;