(XTupdate_begin): Discard records of prior mouse highlight

(XTupdate_begin): Discard records of prior mouse highlight
when frame must be redrawn.

(x_make_frame_visible, x_make_frame_invisible)
(x_iconify_frame): Give error if frame has explicit_parent.
(x_new_font): Don't set window height if frame has explicit_parent.

(x_mouse_leave): New function broken out of XTread_socket.
When x_focus_event_frame is nonzero, assume that frame gets focus.
(XTread_socket): Use x_mouse_leave for LeaveNotify.