OsSetTerrainTextures

• a set of texture keys for legacy viewers and map
• a set of keys of textures or pbr materials for new viewers.
  

so this function sets terrain:

• textures for legacy viewers and map if types == 0 or 2
• textures for new viewers if types == 1 or 2
• PBR materials for new viewers if types == 1
  

The list textureKeys must contain keys or names of 4 textures or 4 pbr materials. They must be of the same type due to viewers restrictions. If the entry is a name, then the material or texture must be present on the prim inventory.