backend/libinput: ignore multiple events for same pointer button

If the same button is pressed on two devices on the same seat,
ignore the second event.

This is also what Mutter does.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3665

backend/libinput/pointer.c

@@ -63,12 +63,22 @@ void handle_pointer_button(struct libinput_event *event,
 	wlr_event.time_msec =
 		usec_to_msec(libinput_event_pointer_get_time_usec(pevent));
 	wlr_event.button = libinput_event_pointer_get_button(pevent);
+	// Ignore events which aren't a seat-wide state change. For instance, if
+	// the same button is pressed twice on the same seat, ignore the second
+	// press.
+	uint32_t seat_count = libinput_event_pointer_get_seat_button_count(pevent);
 	switch (libinput_event_pointer_get_button_state(pevent)) {
 	case LIBINPUT_BUTTON_STATE_PRESSED:
 		wlr_event.state = WLR_BUTTON_PRESSED;
+		if (seat_count != 1) {
+			return;
+		}
 		break;
 	case LIBINPUT_BUTTON_STATE_RELEASED:
 		wlr_event.state = WLR_BUTTON_RELEASED;
+		if (seat_count != 0) {
+			return;
+		}
 		break;
 	}
 	wl_signal_emit_mutable(&pointer->events.button, &wlr_event);