widget: gtk: Pass NULL for geometry widget
widget: gtk: Pass NULL for geometry widget

Since gtk 3.20 the geometry hints are not applied if
the geometry widget passed is not NULL.

This makes geometry a bit better, but still not right. Need
to bring over the fixed to gnome-terminal from bug 760944.

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