[MC-125037] Biomes on land and landscape changed in new world generator

WAI reason

We wanted to do changes to the worldgen system to make it simple/clear and possible to datadrive it in the future.

While doing so we noticed at a certain moment that we generated some areas of the terrain differently than before (not meaning the things placed as decoration, but the actual shape of the land). It took a while to figure out why, but it turned out that in the old code, there had been a bug from the beginning. We forgot to reset the seed of a random when we merged rivers and the biomes.

In the new code there was no way to reproduce this wrong behavior and this is why [this issue] has been resolved as Working as Intended.

We're not happy that we ended up changing the world where the intention was not to. We found no way to keep the bug while also keeping the new functionality we've added. So we chose to keep the new bug-free implementation and hopefully be able to offer datadriven biome definitions in the future because of it.

– [Mojang] Grum (Erik Broes) in this Reddit comment

The new world generator changed some biome and landscape generation compared to 1.12.2. Examples:

Changed terrain and missing ice spikes and hill of flower forest biomes (Seed 5178019746038571832, /tp @p 340 104 -3085 40 20)

Missing Desert M and Sunflower Plains biomes (Seed 1443894742985852992, /tp @p 100 200 100 0 90)

Changed terrain and new Savanna M biome (Seed 1443894742985852992, /tp @p 195 115 555 60 27)

Missing flower forest (Seed 554762737523904, /tp @p 394 118 -353)