evdev: Flush motion events when the slot changes, not just after sync

If two fingers are released almost simultaneously then evdev can send
the touch up events in one bunch without sending a sync event
in-between. However, the evdev_device struct only keeps track of one
pending touch up event so in this case the second touch up event would
override the first and it would be lost. This patch changes it to also
flush the events whenever the slot changes so that it will flush the
previous touch up event before trying to queue the next one.

https://bugs.freedesktop.org/show_bug.cgi?id=67563

src/evdev-touchpad.c

@@ -493,7 +493,7 @@ touchpad_update_state(struct touchpad_dispatch *touchpad, uint32_t time)
 			touchpad->device->rel.dx = wl_fixed_from_double(dx);
 			touchpad->device->rel.dy = wl_fixed_from_double(dy);
 			touchpad->device->pending_events |=
-				EVDEV_RELATIVE_MOTION | EVDEV_SYN;
+				EVDEV_RELATIVE_MOTION | EVDEV_SYN_OR_SLOT;
 		} else if (touchpad->finger_state == TOUCHPAD_FINGERS_TWO) {
 			if (dx != 0.0)
 				notify_axis(touchpad->device->seat,

src/evdev.c

@@ -110,6 +110,7 @@ evdev_process_touch(struct evdev_device *device, struct input_event *e)
 	switch (e->code) {
 	case ABS_MT_SLOT:
 		device->mt.slot = e->value;
+		device->pending_events |= EVDEV_SYN_OR_SLOT;
 		break;
 	case ABS_MT_TRACKING_ID:
 		if (e->value >= 0)
@@ -261,11 +262,11 @@ evdev_flush_motion(struct evdev_device *device, uint32_t time)
        int32_t cx, cy;
        int slot;
 
-       if (!(device->pending_events & EVDEV_SYN))
+       if (!(device->pending_events & EVDEV_SYN_OR_SLOT))
 		return;
 
 	slot = device->mt.slot;
-	device->pending_events &= ~EVDEV_SYN;
+	device->pending_events &= ~EVDEV_SYN_OR_SLOT;
 	if (device->pending_events & EVDEV_RELATIVE_MOTION) {
 		notify_motion(master, time, device->rel.dx, device->rel.dy);
 		device->pending_events &= ~EVDEV_RELATIVE_MOTION;
@@ -332,7 +333,7 @@ fallback_process(struct evdev_dispatch *dispatch,
 		evdev_process_key(device, event, time);
 		break;
 	case EV_SYN:
-		device->pending_events |= EVDEV_SYN;
+		device->pending_events |= EVDEV_SYN_OR_SLOT;
 		break;
 	}
 }

src/evdev.h

@@ -36,7 +36,7 @@ enum evdev_event_type {
 	EVDEV_ABSOLUTE_MT_MOTION = (1 << 2),
 	EVDEV_ABSOLUTE_MT_UP = (1 << 3),
 	EVDEV_RELATIVE_MOTION = (1 << 4),
-	EVDEV_SYN = (1 << 5),
+	EVDEV_SYN_OR_SLOT = (1 << 5),
 };
 
 enum evdev_device_capability {