Cavegen: Fix errors when getting biome outside mapchunk (#7480)

Cavegen: Fix errors when getting biome outside mapchunk (#7480)

Some cave segments are outside the mapchunk.

Previously, biome was being calculated by a function that uses the noise
maps. Points outside the mapchunk resulted in incorrect noise map indexes
that were sometimes outside the noise map size, causing a crash.

Use either noise maps or point noise calculations depending on point
location.