Selection border should not overlap the selection

Currently, slurp centers the border on the edge of the selection, rather than pushing it to the outside. This is especially noticeable when piping in predefined areas for selecting whole windows. This obscures the selected content when using thicker borders, and it's a bit confusing.
Both [slop](https://github.com/naelstrof/slop) and [hacksaw](https://github.com/neXromancers/hacksaw) draw the border around the selection with no overlap.