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	modifiers containing a Meta_ keysym, use the Alt keysyms to
	denote meta.
	(construct_mouse_click): Set the down_modifier bit on mouse
	button press events.
	(XTread_socket): When processing keypress events, use
	x_meta_mod_mask when processing ordinary ASCII characters, not
	just when processing function keys and other non-ASCII events.
	(XTread_socket): If we receive a MappingNotify event with the
	`request' member set to `MappingModifier', then call
	x_find_modifier_meanings to refresh x_meta_mod_mask.