Better in city check

[Procyonae]

Summary

None

Purpose of change

Fixes #74177
Might be used for #71822 and the first commit is code from it mainly courtesy of anothersimulacrum

Describe the solution

Current checks for whether stuff is in a city or not use the difference between the test points distance from the nearest city's centre and the nearest city's size. Given that our cities are thankfully more organic than perfect circles that isn't particularly accurate.
Now every city road and building that gets placed is added to a set of point_om_omts per overmap that can be checked against to determine if you're in a city. Once all cities are placed in an overmap the result is floodfilled so random fields in the middle of cities still count as being in the city limits.
Currently only affects EOCs map_in_city and overmap_specials city_distance
Falls back on calculating distance based on city size but cuts off at 0 as < 0 isn't a useful result

Describe alternatives you've considered

Keeping track of tripoints instead of just points but that seems excessive
Adding 1 tile padding to the edges of city_tiles after it's flood filled
Adding more initial usages but this has been sitting drafted for ages

Testing

For basic city_tiles testing or map_in_city testing plop this anywhere in the JSON and walk around, also tested post save/load to check that works

  {
    "type": "effect_on_condition",
    "id": "EOC_TEST_IS_IN_CITY",
    "eoc_type": "EVENT",
    "required_event": "avatar_enters_omt",
    "condition": { "map_in_city": { "mutator": "u_loc_relative", "target": "(0,0,0)" } },
    "effect": [ { "u_message": "You are in a city OMT.", "type": "good" } ],
    "false_effect": [ { "u_message": "You are NOT in a city OMT.", "type": "bad" } ]
  },

For city_distance performance testing add this to a test cpp and run it with and without the changes, comparing the times

TEST_CASE( "overmap_generation_time", "[overmap]" )
{
    overmap_buffer.clear();
    const point_abs_om origin;
    for( const point_abs_om &overmap_point : closest_points_first( origin, 5 ) ) {
        overmap &om = overmap_buffer.get( overmap_point );
    }
}

Additional context