widget: Allow calling get_char_width/height on unrealized widget
widget: Allow calling get_char_width/height on unrealized widget

This isn't really ok, but somehow it's happening in gnome-terminal.

https://bugzilla.gnome.org/show_bug.cgi?id=758841

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