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