Correct occlusion distance calc for non-isometric tilesets

[Procyonae]

Summary

None

Purpose of change

Describe the solution

Describe alternatives you've considered

Testing

Used this test map of 3x3 OMTs filled with casings

  {
    "type": "mapgen",
    "method": "json",
    "om_terrain": [ "casings" ],
    "object": { "fill_ter": "t_region_groundcover", "item": { " ": { "item": "4570_casing", "chance": 100 } } }
  },
  {
    "type": "overmap_terrain",
    "id": "casings",
    "name": "nothing",
    "sym": "%",
    "color": "white",
    "flags": [ "NO_ROTATE" ]
  },
  {
    "type": "overmap_special",
    "id": "casings",
    "overmaps": [
      { "point": [ 0, 0, 0 ], "overmap": "casings" },
      { "point": [ 1, 0, 0 ], "overmap": "casings" },
      { "point": [ 2, 0, 0 ], "overmap": "casings" },
      { "point": [ 0, 1, 0 ], "overmap": "casings" },
      { "point": [ 0, 2, 0 ], "overmap": "casings" },
      { "point": [ 1, 1, 0 ], "overmap": "casings" },
      { "point": [ 2, 1, 0 ], "overmap": "casings" },
      { "point": [ 1, 2, 0 ], "overmap": "casings" },
      { "point": [ 2, 2, 0 ], "overmap": "casings" }
    ],
    "locations": [ "field" ],
    "occurrences": [ 0, 8 ]
  }

Max occlusion = 10:

Max occlusion = 16:

Iso occlusion seems to creep off centre starting at 16 but that's not what I'm here to solve rn

Additional context