Update layer shell with optional wl_output

include/wlr/types/wlr_layer_shell.h

@@ -27,6 +27,9 @@ struct wlr_layer_shell {
 	struct wl_listener display_destroy;
 
 	struct {
+		 // struct wlr_layer_surface *
+		 // Note: the output may be NULL. In this case, it is your
+		 // responsibility to assign an output before returning.
 		struct wl_signal new_surface;
 	} events;
 

protocol/wlr-layer-shell-unstable-v1.xml

@@ -47,12 +47,16 @@
         or manipulate a buffer prior to the first layer_surface.configure call
         must also be treated as errors.
 
+        You may pass NULL for output to allow the compositor to decide which
+        output to use. Generally this will be the one that the user most
+        recently interacted with.
+
         Clients can specify a namespace that defines the purpose of the layer
         surface.
       </description>
       <arg name="id" type="new_id" interface="zwlr_layer_surface_v1"/>
       <arg name="surface" type="object" interface="wl_surface"/>
-      <arg name="output" type="object" interface="wl_output"/>
+      <arg name="output" type="object" interface="wl_output" allow-null="true"/>
       <arg name="layer" type="uint" enum="layer" summary="layer to add this surface to"/>
       <arg name="namespace" type="string" summary="namespace for the layer surface"/>
     </request>

types/wlr_layer_shell.c

@@ -319,7 +319,9 @@ static void layer_shell_handle_get_layer_surface(struct wl_client *wl_client,
 
 	surface->shell = shell;
 	surface->surface = wlr_surface;
+	if (output_resource) {
 		surface->output = wlr_output_from_resource(output_resource);
+	}
 	surface->resource = wl_resource_create(wl_client,
 		&zwlr_layer_surface_v1_interface,
 		wl_resource_get_version(client_resource),