Access overmap terrain by global coordinates, follow-up to #5619

Changes in overmap::draw_overmap:
- Timeout for blinking is set before calling draw and reset directly after draw returns. This allows the other parts of the function to request input without disabling and re-enabling the timeout.
- It returns global absolute overmap terrain coordinate. This requires the few places where this is used to be changed, too.
- game::nuke: takes global overmap terrain coordinates, does work on any overmap now.

Changes in overmap::draw:
- Request overmap terrain only if the location can be seen. As the player-has-seen status is recorded in the overmap itself, I had to make sure that any overmap that exist on disk gets loaded when requested. This prevents new overmaps from being created when the PC can't actually see them.
- display priority changed: player-pos* < mission-target* < note* < invisible-terrain < npcs* < vehicles* < terrain (*only when blinking)
It used to be invisible-terrain < note* < player-pos* < npcs* < vehicles* < mission-target* < terrain. If this is not good, the priority can be easily changed by moving the `} else if(...) { ... }` blocks around.
- I was confused about the `ch` parameter, it seems to make the height of the drawn area increase in one case, but that only if the player previously moved the view downwards. And it would get reset to ERR/TIMEOUT because of the blinking anyway.

overmap::find_note:
- grep all notes of the current z-level in all overmaps

If this changes to much, let me know and I'll change it.