Mapgen: Use decoration sidelen 16 for jungletrees and junglegrass

Currently jungletrees and junglegrass use sidelen 80 for simplicity,
but this results in a more uneven distribution of decorations. A more
even distribution helps keep rainforest darker with a more unbroken
canopy.
This is also more consistent. 80 is based on the default mapchunk
size, all other decorations use sidelen 16 or smaller to divide into
any mapchunk size.

mods/default/mapgen.lua

@@ -1324,7 +1324,7 @@ function default.register_decorations()
 	minetest.register_decoration({
 		deco_type = "schematic",
 		place_on = {"default:dirt_with_rainforest_litter", "default:dirt"},
-		sidelen = 80,
+		sidelen = 16,
 		fill_ratio = 0.1,
 		biomes = {"rainforest", "rainforest_swamp"},
 		y_min = -1,
@@ -1337,7 +1337,7 @@ function default.register_decorations()
 	minetest.register_decoration({
 		deco_type = "schematic",
 		place_on = {"default:dirt_with_rainforest_litter", "default:dirt"},
-		sidelen = 80,
+		sidelen = 16,
 		fill_ratio = 0.005,
 		biomes = {"rainforest", "rainforest_swamp"},
 		y_min = 1,
@@ -1598,7 +1598,7 @@ function default.register_decorations()
 	minetest.register_decoration({
 		deco_type = "simple",
 		place_on = {"default:dirt_with_rainforest_litter"},
-		sidelen = 80,
+		sidelen = 16,
 		fill_ratio = 0.1,
 		biomes = {"rainforest"},
 		y_min = 1,