Add wrapper function to access overmap terrain by global coordinates

This PR replace #5593, which is faulty and bad structured. This branch is ready to be merged, further additions will go into another branch.

The final goal of this is to replace access to overmaps with access to the overmapbuffer instead. That buffer is responsible for choosing and loading/generating of the overmap.

This adds:
- access to the overmap terrain
- access to the visibility of overmap terrain (known by PC)
- notes, has_vehicle, has_npc (used for drawing the overmap)
- reveal function to set area to be known by the PC

Changes:
- removes om_hori, om_vert, om_diag and game::set_adjacent_overmaps.
- uses reveal function instead of loops

TODO:
- Display/add/remove notes in unexplored terrain.
- Reformat overmap::draw and make it static, as it does not need a specific overmap.
- Search for notes in all overmaps.
- Missions & NPC vs current overmap (mission targets are local to the current overmap), load npcs from adjacent overmaps.
- replace access to cur_om with access to overmap_buffer.
- Vehicle tracker.
- access radio towers across overmap borders