Set grid shape to convex polygon

objects/Grid.cs

@@ -116,17 +116,26 @@ public partial class Grid : Area3D
 		return _material;
 	}
 
-	BoxShape3D _shape;
+	ConvexPolygonShape3D _shape;
 	void UpdateCollisionShape()
 	{
 		if (Engine.IsEditorHint()) return;
 
 		if (_shape == null) {
-			_shape = new BoxShape3D();
+			_shape = new ConvexPolygonShape3D();
 			AddChild(new CollisionShape3D { Shape = _shape }, true);
 		}
 
-		_shape.Size = new(GridSize.X * StepSize, 0.001f, GridSize.Y * StepSize);
+		const float Offset = 0.001f;
+		var x = GridSize.X * StepSize / 2;
+		var y = GridSize.Y * StepSize / 2;
+
+		_shape.Points = [
+			new(-x, Offset, -y),
+			new( x, Offset, -y),
+			new( x, Offset,  y),
+			new(-x, Offset,  y),
+		];
 	}
 
 	ImmediateMesh _mesh;