Decorations: Add kelp to cool and temperate shallow ocean

Original texture by tobyplowy, colourised by paramat.

mods/default/mapgen.lua

@@ -2019,6 +2019,37 @@ function default.register_decorations()
 		flags = "place_center_x, place_center_z",
 		rotation = "random",
 	})
+
+	-- Kelp
+
+	minetest.register_decoration({
+		deco_type = "simple",
+		place_on = {"default:sand"},
+		place_offset_y = -1,
+		sidelen = 16,
+		noise_params = {
+			offset = -0.04,
+			scale = 0.1,
+			spread = {x = 200, y = 200, z = 200},
+			seed = 87112,
+			octaves = 3,
+			persist = 0.7
+		},
+		biomes = {
+			"taiga_ocean",
+			"snowy_grassland_ocean",
+			"grassland_ocean",
+			"coniferous_forest_ocean",
+			"deciduous_forest_ocean",
+			"sandstone_desert_ocean",
+			"cold_desert_ocean"},
+		y_min = -10,
+		y_max = -5,
+		flags = "force_placement",
+		decoration = "default:sand_with_kelp",
+		param2 = 48,
+		param2_max = 96,
+	})
 end