dix: Correctly save replayed event into GrabInfoRec

When processing events we operate on InternalEvent pointers. They may actually refer to a an instance of DeviceEvent, GestureEvent or any other event that comprises the InternalEvent union. This works well in practice because we always look into event type before doing anything, except in the case of copying the event.

*dst_event = *src_event would copy whole InternalEvent event and would cause out of bounds read in case the pointed to event was not InternalEvent but e.g. DeviceEvent.

This regression has been introduced in 23a8b62d.

cc @whot