Match gnome-terminal with its font scaling logic

Gnome terminal jumps between a predefined set of font scales. libvte seems
to break if we try to jump in between them. This behaviour is an exact
match from what you get in gnome-terminal (and certainly looks
considerably better).

Relevant to issue #195.
Closes #194 since it no longer crashes.
Probably a proper fix (finally) for #47.