revealer: Request child size for NONE transition type

revealer: Request child size for NONE transition type

Previously, we would request a size of 0×0 when the transition type was
NONE and the child un-revealed, making the revealer in this case a
gtk_widget_set_visible replacement. Instead, to the exact same thing we
do in the CROSSFADE case and request the child size instead. This also
keeps the revealer from under allocating the child when the transition
type is set to NONE.