widget: Fix get_preferred_height to use the correct padding
widget: Fix get_preferred_height to use the correct padding

The implementation of GtkWidgetClass:get_preferred_height by
mistake used the horizontal padding instead of the vertical padding.

https://bugzilla.gnome.org/show_bug.cgi?id=760944

thestinger/vte-ng,thestinger/vte-ng,saitoha/vte-sixel,saitoha/vte-sixel,thestinger/vte-ng,saitoha/vte-sixel