widget: Fix the adjustment's upper boundary
widget: Fix the adjustment's upper boundary

This is necessary for the contents to look properly when placed inside a
GtkScrolledWindow.

https://bugzilla.gnome.org/show_bug.cgi?id=757448

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