Added isPayload utility for buildings

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

@@ -978,6 +978,11 @@ abstract class BuildingComp implements Posc, Teamc, Healthc, Buildingc, Timerc,
         }
     }
 
+    /** @return whether this building is in a payload */
+    public boolean isPayload(){
+        return tile == emptyTile;
+    }
+
     /**
      * Called when a block is placed over some other blocks. This seq will always have at least one item.
      * Should load some previous state, if necessary. */