weston-simple-im: Use serial number from incoming events

weston-simple-im: Use serial number from incoming events

The key events we pass through to the input_method_context has to have
a serial number that corresponds to the key event we got.  The struct display
serial is updated on pointer enter/leave and keyboard events, but not the
input method keyboard events.  So the display serial will never correspond
to the key event we're dealing with and we have to pass through the
serial we get from the key event.