It seems Wayland-based window managers provide additional decorations
that we're not tracking in our "preferred size" computations. This means
every geometry update shrinks the window by a bit.

Factor in the allocated dimensions, not just the preferred ones.

Borrows some ideas from
https://bugzilla.gnome.org/show_bug.cgi?id=769898

I also drop a few unused values leftover from gnome-terminal.

Fixes #350

This fixes an issue when a monospace font is displayed instead of
a proper one in mate-terminal context (right-click) menu.

This fixes: https://github.com/mate-desktop/mate-terminal/issues/407