Conditional nesting in mapgen - by overmap neighbor adjacency

[Coolthulhu]

A step in the direction of jsonized labs. Still not there yet, but not far from it.

A nested entry now has an optional "neighbors" field. If the field exists, the nested mapgen will only be applied if all the neighbors match the ones specified there. For example:

{
  "chunks": [ "gun_with_casings", "shrubs_3x3" ],
  x: [ 10, 20 ],
  y: [ 10, 20 ],
  "neighbors": { "north": [ "forest", "field" }, "south": [ "forest", "field" ] }
}

In this case one of the two nested mapgens will be applied at random point in square from [ 10, 10 ] to [ 20, 20 ] only if north is a forest or field and south is a forest or field (can be mixed, like north being forest and south being field).

I edited the swamp_shack.json to show this new feature. The changes are as follows:

• Moved the shack to center of the block, like most structures
• Removed random chance of cattail on mixed terrain (they should only grow on shallow water)
• Each side (north/east/south/west) gets an average of 2.5 cattails (in shallow, fresh water) if that side is adjacent to a swamp

Additionally, minor tweaks:

• Supply const mapgendata & instead of map * to json mapgen pieces. That structure is already created in mapgen above and it usually goes to waste. This structure contains adjacency data (for the feature here), but also "biome" data. Biomes are nowhere near functional, but having this data would make it somewhat more likely that this will move somewhere sometime.
• Minor refactor of mapgendata structure.

[John-Candlebury]

What does the null mean in here?

[Coolthulhu]

If the null is rolled, nothing is nested. They have equal weights. So 50% chance for cattail.
This is to pad the chances a bit, to make it somewhat more like the surrounding forest. Doing this whole "x rolls for cattail, 50% chance" makes it more like the math in forest cattail placement than just rolling "1 to 4 cattails" and placing those.