output-layers: change semantics of wlr_output_state.layers

Previously, we were requiring all layers to be included in wlr_output_state.layers and wlr_output_layer_state.buffer to be set to NULL to disable a layer.

This commit changes these semantics: disabled layers are left out of wlr_output_state.layers, and wlr_output_layer_state.buffer is required to be non-NULL.

This new API should make it easier for callers to populate the layers, at the cost of some additional complexity in backends, mostly addressed by introducing a new wlr_output_for_each_disabled_layer() helper.

Depends on !4194 (merged)

cc @Nefsen402