backend/x11: ensure buffers are released on shutdown

backend/x11/output.c

@@ -145,6 +145,9 @@ static void destroy_x11_buffer(struct wlr_x11_buffer *buffer) {
 	wl_list_remove(&buffer->buffer_destroy.link);
 	wl_list_remove(&buffer->link);
 	xcb_free_pixmap(buffer->x11->xcb, buffer->pixmap);
+	for (size_t i = 0; i < buffer->n_busy; i++) {
+		wlr_buffer_unlock(buffer->buffer);
+	}
 	free(buffer);
 }
 
@@ -268,6 +271,7 @@ static struct wlr_x11_buffer *get_or_create_x11_buffer(
 	wl_list_for_each(buffer, &output->buffers, link) {
 		if (buffer->buffer == wlr_buffer) {
 			wlr_buffer_lock(buffer->buffer);
+			buffer->n_busy++;
 			return buffer;
 		}
 	}
@@ -679,6 +683,8 @@ void handle_x11_present_event(struct wlr_x11_backend *x11,
 			return;
 		}
 
+		assert(buffer->n_busy > 0);
+		buffer->n_busy--;
 		wlr_buffer_unlock(buffer->buffer); // may destroy buffer
 		break;
 	case XCB_PRESENT_COMPLETE_NOTIFY:;

include/backend/x11.h

@@ -113,6 +113,7 @@ struct wlr_x11_buffer {
 	xcb_pixmap_t pixmap;
 	struct wl_list link; // wlr_x11_output::buffers
 	struct wl_listener buffer_destroy;
+	size_t n_busy;
 };
 
 struct wlr_x11_format {