Biome API: Fix absent water decorations and dust, in deep water (#7470)
Previously, the biomemap was 'BIOME_NONE' for a mapchunk column if a
stone surface was not found in it, causing water surface decorations
and water surface dust to fail.
Store the biome ID of the biome calculated at a water surface and add
it to the biomemap if the biomemap is 'BIOME_NONE' for the mapchunk
column. The biome calculated at a stone surface still has priority
for the biomemap entry, as it should.
Edit an incorrect comment.