Toytoolkit doesn't buy us anything in this case, we're not rendering or
handling regular input events. Just talk directly to wl_display and
look up the 'input_method' global directly.
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.