- Simplifies the backends
- Avoids having two ways to do the same thing: previously one could
disable a layer by either omitting it from wlr_output_state.layers,
or by passing a NULL buffer
- We can change our mind in the future: we can allow users to omit
some layers and define a meaning without breaking the API.
References: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4017#note_1783997