backend: unbreak on 32-bit architectures

backend/headless/output.c:132:3: error: format specifies type 'long' but the argument has type 'size_t' (aka 'unsigned int') [-Werror,-Wformat]
                ++backend->last_output_num);
                ^~~~~~~~~~~~~~~~~~~~~~~~~~
backend/noop/output.c:72:3: error: format specifies type 'long' but the argument has type 'size_t' (aka 'unsigned int') [-Werror,-Wformat]
                ++backend->last_output_num);
                ^~~~~~~~~~~~~~~~~~~~~~~~~~
backend/wayland/output.c:294:3: error: format specifies type 'unsigned long' but the argument has type 'size_t' (aka 'unsigned int') [-Werror,-Wformat]
                ++backend->last_output_num);
                ^~~~~~~~~~~~~~~~~~~~~~~~~~
backend/x11/output.c:150:3: error: format specifies type 'long' but the argument has type 'size_t' (aka 'unsigned int') [-Werror,-Wformat]
                ++x11->last_output_num);
                ^~~~~~~~~~~~~~~~~~~~~~

backend/headless/output.c

@@ -128,7 +128,7 @@ struct wlr_output *wlr_headless_add_output(struct wlr_backend *wlr_backend,
 	output_set_custom_mode(wlr_output, width, height, 0);
 	strncpy(wlr_output->make, "headless", sizeof(wlr_output->make));
 	strncpy(wlr_output->model, "headless", sizeof(wlr_output->model));
-	snprintf(wlr_output->name, sizeof(wlr_output->name), "HEADLESS-%ld",
+	snprintf(wlr_output->name, sizeof(wlr_output->name), "HEADLESS-%zd",
 		++backend->last_output_num);
 
 	if (!wlr_egl_make_current(&output->backend->egl, output->egl_surface,

backend/noop/output.c

@@ -68,7 +68,7 @@ struct wlr_output *wlr_noop_add_output(struct wlr_backend *wlr_backend) {
 
 	strncpy(wlr_output->make, "noop", sizeof(wlr_output->make));
 	strncpy(wlr_output->model, "noop", sizeof(wlr_output->model));
-	snprintf(wlr_output->name, sizeof(wlr_output->name), "NOOP-%ld",
+	snprintf(wlr_output->name, sizeof(wlr_output->name), "NOOP-%zd",
 		++backend->last_output_num);
 
 	wl_list_insert(&backend->outputs, &output->link);

backend/wayland/output.c

@@ -290,7 +290,7 @@ struct wlr_output *wlr_wl_output_create(struct wlr_backend *wlr_backend) {
 	wlr_output_update_custom_mode(wlr_output, 1280, 720, 0);
 	strncpy(wlr_output->make, "wayland", sizeof(wlr_output->make));
 	strncpy(wlr_output->model, "wayland", sizeof(wlr_output->model));
-	snprintf(wlr_output->name, sizeof(wlr_output->name), "WL-%lu",
+	snprintf(wlr_output->name, sizeof(wlr_output->name), "WL-%zd",
 		++backend->last_output_num);
 
 	output->backend = backend;

backend/x11/output.c

@@ -146,7 +146,7 @@ struct wlr_output *wlr_x11_output_create(struct wlr_backend *backend) {
 
 	output_set_refresh(&output->wlr_output, 0);
 
-	snprintf(wlr_output->name, sizeof(wlr_output->name), "X11-%ld",
+	snprintf(wlr_output->name, sizeof(wlr_output->name), "X11-%zd",
 		++x11->last_output_num);
 	parse_xcb_setup(wlr_output, x11->xcb);