output: allow modeset with buffer while frame is pending

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3600

types/output/output.c

@@ -576,7 +576,10 @@ static uint32_t output_compare_state(struct wlr_output *output,
 static bool output_basic_test(struct wlr_output *output,
 		const struct wlr_output_state *state) {
 	if (state->committed & WLR_OUTPUT_STATE_BUFFER) {
-		if (output->frame_pending) {
+		// Modesets will block for the previous frame to complete. Regular
+		// page-flips are non-blocking and require the compositor to wait.
+		if (output->frame_pending &&
+				!(state->committed & (WLR_OUTPUT_STATE_ENABLED | WLR_OUTPUT_STATE_MODE))) {
 			wlr_log(WLR_DEBUG, "Tried to commit a buffer while a frame is pending");
 			return false;
 		}