Makes layer the current layer while callback runs. It is also remembered as the fallback layer.
bind: (layer: Instance?) -> ()
Sets the fallback layer, used by components that are created when there is no current layer. A Window calls it for its own overlay layer. Pass nil to clear it.
Parents instance to the resolved layer, and removes it again when the scope is cleaned up. It returns false, and leaves instance alone, when there is no layer.