output: fix dangling renderer context after wlr_output_preferred_read_format

attach_render was called without un-setting the current rendering
context afterwards.

Closes: https://github.com/swaywm/wlroots/issues/2164

types/wlr_output.c

@@ -448,15 +448,16 @@ bool wlr_output_attach_render(struct wlr_output *output, int *buffer_age) {
 
 bool wlr_output_preferred_read_format(struct wlr_output *output,
 		enum wl_shm_format *fmt) {
-	if (!output->impl->attach_render(output, NULL)) {
+	struct wlr_renderer *renderer = wlr_backend_get_renderer(output->backend);
+	if (!renderer->impl->preferred_read_format || !renderer->impl->read_pixels) {
 		return false;
 	}
 
-	struct wlr_renderer *renderer = wlr_backend_get_renderer(output->backend);
-	if (!renderer->impl->preferred_read_format || !renderer->impl->read_pixels) {
+	if (!output->impl->attach_render(output, NULL)) {
 		return false;
 	}
 	*fmt = renderer->impl->preferred_read_format(renderer);
+	output->impl->rollback_render(output);
 	return true;
 }