Overmap overhaul (step 8)

[codemime]

Continues #21563.
One step closer to #19298 (see my comments there).

Observable behavior

• Roads try to avoid the terrain where it's more difficult to pave them (rivers, swamps). The difficulty can be explicitly specified.
• More natural curvature of roads by virtue of the tweaked "estimator" function. Apart from the weights assigned to different road types, it uses square distance instead of linear distance.

Note how the roads reasonably avoid swamps and forests:

The swampy bank of the river isn't a good place for the road, so it goes around:

Terrain-specific road types can now be defined:

• bridges for rivers (have always been there, but now there's nothing special about them - just yet another connection for the river locations).

What can be done in the future:

• shady roads for thick forests (fallen trees would spawn on them).
• watery roads for swamps (puddles and stuff).

Implementation details

• Overmap connections were JSONized too (and encapsulated into their own classes).
• There's no need to polish any connections (roads, sewers, subway tracks, ant tunnels, e.t.c). They are built perfectly connected initially. So the polish function now governs only rivers (5 lines of code). This is probably faster, but I didn't measure the difference.
• Highways await for proper implementation.
• There's a new flag ORTHOGONAL which produces more angled connections (with fewer turns). It's used to make subway and sewer tunnels.
• The flag ALLOW_OVERRIDE was, in turn, removed. There's no need for it anymore: the connections themselves "know" which locations they can/cannot override.
• Simple pathfinding now produces path objects (the class will be extended later on).
• "Planning" the connections (lay_out_connection) now precedes the actual building of them (build_connection).
• Opposite direction of paths (from specials to cities, not vice versa). This will be useful to make country roads (that lead from cabins and farms).
• Minor code improvements.

[vache]

In the first example, the roads on the left side of the river don't connect to the right side at all, except to dead end at specials. May be outside the scope of this PR, but they should probably always try to connect across the dividing river.

[codemime]

They probably do connect (down there, beyond the lower edge of the screen). However, connecting dead ends that are close enough to each other is a good idea.

[Coolthulhu]

The flag ALLOW_OVERRIDE was, in turn, removed. There's no need for it anymore: the connections themselves "know" which locations they can/cannot override.

Does it cover all cases?
For example, "ranch_camp_77" loses the flag, but it doesn't seem to be in any of the groups.

[Coolthulhu]

Are you using it as non-const? Would be better to scrap the non-const getters and use setters instead, to make more proper encapsulation possible.

[codemime]

Agreed, but I didn't invent this approach here, merely added an overloaded version of the function. I guess, references could improve performance under certain conditions, since both getting and setting the value perform the same steps which involve checking bounds and calculating indexes. However, the impact is probably unnoticeable.

[codemime]

Does it cover all cases? For example, "ranch_camp_77" loses the flag, but it doesn't seem to be in any of the groups.

Good catch! I've missed that. Will fix.

[acidia]

Having strange behavior in #21809, subway connections in specials are being connected with sewer lines instead. I think I tracked it down to overmap_connections::guess_for()... It is returning the first overmap_connection entry that is underground? I simply reversed the order of subway_tunnel and sewer_tunnel in the json and it started working as intended.