Allow placing floors (for mods)

core/src/mindustry/world/Build.java

@@ -169,6 +169,10 @@ public class Build{
             return false;
         }
 
+        if(type.isFloor() && tile.floor == type){
+            return false;
+        }
+
         if(!type.canPlaceOn(tile, team, rotation)){
             return false;
         }

core/src/mindustry/world/Tile.java

@@ -303,7 +303,9 @@ public class Tile implements Position, QuadTreeObject, Displayable{
     public void setFloorUnder(Floor floor){
         Block overlay = this.overlay;
         setFloor(floor);
-        setOverlay(overlay);
+        if(this.overlay != overlay){
+            setOverlay(overlay);
+        }
     }
 
     /** Sets the block to air. */

core/src/mindustry/world/blocks/ConstructBlock.java

@@ -20,6 +20,7 @@ import mindustry.graphics.*;
 import mindustry.logic.*;
 import mindustry.type.*;
 import mindustry.world.*;
+import mindustry.world.blocks.environment.*;
 import mindustry.world.blocks.storage.CoreBlock.*;
 import mindustry.world.modules.*;
 
@@ -72,7 +73,12 @@ public class ConstructBlock extends Block{
         float healthf = tile.build == null ? 1f : tile.build.healthf();
         Seq<Building> prev = tile.build instanceof ConstructBuild co ? co.prevBuild : null;
 
-        tile.setBlock(block, team, rotation);
+        if(block instanceof Floor floor){
+            tile.setFloorUnder(floor);
+            tile.setBlock(Blocks.air);
+        }else{
+            tile.setBlock(block, team, rotation);
+        }
 
         if(tile.build != null){
             tile.build.health = block.health * healthf;