use the correct overmap on add/delete/change notes

fix for #4292

Much of `overmap::draw` relies on the `this` being the overmap that contains the center of the view. `draw` is only called from `draw_overmap` and always with the current overmap (where the player is on) as `this`. This does not even change when the user scrolls far away.

As a result in `draw`, the other three overmaps (`hori`, `vert` and `diag`) get updated but the `this` overmap not. As the whole add/delete/change note code is in `draw_overmap` and uses `this` all notes are stored in the overmap the player is currently on (`game::cur_om` called from `game::draw_overmap`). That also means one could only search/list notes on the current overmap.

This PR changes it use the overmap that is currently centered in the view.