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: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4017#note_1783997

include/wlr/types/wlr_output_layer.h

@@ -36,6 +36,8 @@
  * Callers are responsible for disabling output layers when they need the full
  * output contents to be composited onto a single buffer, e.g. during screen
  * capture.
+ *
+ * Callers must always include the state for all layers on output test/commit.
  */
 struct wlr_output_layer {
 	struct wl_list link; // wlr_output.layers