terminal: Avoid too large character grid when resized

When resizing the terminal, the row/columns would end up potentially too tall
and/or wide, meaning the widget would grow each time the window was configured
with a size.

Fix this by making sure the calculated rows and columns don't loose too much
precision, and if they do, shrink instead of grow, as that is expected by the
xdg_toplevel configure event.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>