c2129c18f8 saves the button down serial to
pointer_serial of the seat so that it can be used for window dragging
later. However, this overwrites the serial saved at the enter event.
Since the serial in wl_pointer_set_cursor must be the latest
wl_pointer_enter serial number sent to the client, if a button down
serial overwrites that, setting cursor no longer works until the cursor
enters the window next time.
Fix this by using separate serials for these two types of events.
9c03b7569b