output-layer: require all layers in wlr_output_state.layers
- 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: !4017 (comment 1783997)