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.

src/mapgen/mapgen.cpp

@@ -655,6 +655,7 @@ void MapgenBasic::generateBiomes()
	for (s16 z = node_min.Z; z <= node_max.Z; z++)
	for (s16 x = node_min.X; x <= node_max.X; x++, index++) {
		Biome *biome = NULL;
		biome_t water_biome_index = 0;
		u16 depth_top = 0;
		u16 base_filler = 0;
		u16 depth_water_top = 0;
@@ -697,6 +698,11 @@ void MapgenBasic::generateBiomes()
				if (biomemap[index] == BIOME_NONE && is_stone_surface)
					biomemap[index] = biome->index;

				// Store biome of first water surface detected, as a fallback
				// entry for the biomemap.
				if (water_biome_index == 0 && is_water_surface)
					water_biome_index = biome->index;

				depth_top = biome->depth_top;
				base_filler = MYMAX(depth_top +
					biome->depth_filler +
@@ -763,6 +769,11 @@ void MapgenBasic::generateBiomes()

			VoxelArea::add_y(em, vi, -1);
		}
		// If no stone surface detected in mapchunk column and a water surface
		// biome fallback exists, add it to the biomemap. This avoids water
		// surface decorations failing in deep water.
 		if (biomemap[index] == BIOME_NONE && water_biome_index != 0)
			biomemap[index] = water_biome_index;
	}
}

src/mapgen/mg_biome.cpp

@@ -194,9 +194,9 @@ BiomeGenOriginal::BiomeGenOriginal(BiomeManager *biomemgr,
	humidmap = noise_humidity->result;

	biomemap = new biome_t[m_csize.X * m_csize.Z];
	// Initialise with the ID of the default biome so that cavegen can get
	// biomes when biome generation (which calculates the biomemap IDs) is
	// disabled.
	// Initialise with the ID of 'BIOME_NONE' so that cavegen can get the
	// fallback biome when biome generation (which calculates the biomemap IDs)
	// is disabled.
	memset(biomemap, 0, sizeof(biome_t) * m_csize.X * m_csize.Z);
}