Prevent wall from being used to block objective

This prevents hero 1 from permanently setting the objective to one side,
making them very OP and the game un-fun.

Any object with collision mask=2 will no longer collide with the
objective.

scenes/objective.tscn

@@ -25,7 +25,7 @@ height = 14.0
 input_ray_pickable = true
 input_capture_on_drag = false
 collision_layer = 1
-collision_mask = 1
+collision_mask = 2
 mode = 0
 mass = 1000.0
 friction = 1.0

scenes/wall.tscn

@@ -16,7 +16,7 @@ extents = Vector3( 1, 1, 0.1 )
 input_ray_pickable = true
 input_capture_on_drag = false
 collision_layer = 1
-collision_mask = 1
+collision_mask = 2
 friction = 1.0
 bounce = 0.0
 constant_linear_velocity = Vector3( 0, 0, 0 )