backend/wayland: take existing wl_display in wlr_wl_backend_create()

This allows compositors to use an existing wl_display, to integrate
wlroots with an existing toolkit.

backend/wayland/backend.c

@@ -529,7 +529,9 @@ static void backend_destroy(struct wlr_backend *backend) {
 	wl_compositor_destroy(wl->compositor);
 	wl_registry_destroy(wl->registry);
 	wl_display_flush(wl->remote_display);
+	if (wl->own_remote_display) {
 		wl_display_disconnect(wl->remote_display);
+	}
 	free(wl);
 }
 
@@ -568,7 +570,7 @@ static void handle_display_destroy(struct wl_listener *listener, void *data) {
 }
 
 struct wlr_backend *wlr_wl_backend_create(struct wl_display *display,
-		const char *remote) {
+		struct wl_display *remote_display) {
 	wlr_log(WLR_INFO, "Creating wayland backend");
 
 	struct wlr_wl_backend *wl = calloc(1, sizeof(*wl));
@@ -585,11 +587,16 @@ struct wlr_backend *wlr_wl_backend_create(struct wl_display *display,
 	wl_list_init(&wl->buffers);
 	wl->presentation_clock = CLOCK_MONOTONIC;
 
-	wl->remote_display = wl_display_connect(remote);
+	if (remote_display != NULL) {
+		wl->remote_display = remote_display;
+	} else {
+		wl->remote_display = wl_display_connect(NULL);
 		if (!wl->remote_display) {
 			wlr_log_errno(WLR_ERROR, "Could not connect to remote display");
 			goto error_wl;
 		}
+		wl->own_remote_display = true;
+	}
 
 	wl->registry = wl_display_get_registry(wl->remote_display);
 	if (!wl->registry) {
@@ -685,7 +692,9 @@ error_registry:
 	}
 	wl_registry_destroy(wl->registry);
 error_display:
+	if (wl->own_remote_display) {
 		wl_display_disconnect(wl->remote_display);
+	}
 error_wl:
 	wlr_backend_finish(&wl->backend);
 	free(wl);

include/backend/wayland.h

@@ -30,6 +30,7 @@ struct wlr_wl_backend {
 
 	/* remote state */
 	struct wl_display *remote_display;
+	bool own_remote_display;
 	struct wl_event_source *remote_display_src;
 	struct wl_registry *registry;
 	struct wl_compositor *compositor;

include/wlr/backend/wayland.h

@@ -12,12 +12,11 @@ struct wlr_input_device;
  * Creates a new Wayland backend. This backend will be created with no outputs;
  * you must use wlr_wl_output_create() to add them.
  *
- * The `remote` argument is the name of the host compositor wayland socket. Set
- * to NULL for the default behaviour (WAYLAND_DISPLAY env variable or wayland-0
- * default).
+ * The remote_display argument is an existing libwayland-client struct wl_display
+ * to use. Leave it NULL to create a new connection to the compositor.
  */
 struct wlr_backend *wlr_wl_backend_create(struct wl_display *display,
-		const char *remote);
+		struct wl_display *remote_display);
 
 /**
  * Returns the remote struct wl_display used by the Wayland backend.