Add workaround to fix broken double plants in v6

mods/MAPGEN/mcl_mapgen_core/init.lua

@@ -1729,12 +1729,13 @@ minetest.register_on_generated(function(minp, maxp, seed)
 		----- The section to perform basic block overrides of the core mapgen generated world. -----
 
 		-- Snow and sand fixes. This code implements snow consistency
-		-- and fixes floating sand.
+		-- and fixes floating sand and cut plants.
 		-- A snowy grass block must be below a top snow or snow block at all times.
-		if minp.y <= mcl_vars.mg_overworld_max and maxp.y >= mcl_vars.mg_overworld_min then
+		if emin.y <= mcl_vars.mg_overworld_max and emax.y >= mcl_vars.mg_overworld_min then
 			-- v6 mapgen:
 			-- Put top snow on snowy grass blocks. The mapgen does not generate the top snow on its own.
 			if mg_name == "v6" then
+				-- FIXME: Cavegen and mudflow might screw this up and cause floating top snow to appear
 				local snowdirt = minetest.find_nodes_in_area_under_air(minp, maxp, "mcl_core:dirt_with_grass_snow")
 				for n = 1, #snowdirt do
 					-- CHECKME: What happens at chunk borders?
@@ -1743,10 +1744,38 @@ minetest.register_on_generated(function(minp, maxp, seed)
 						data[p_pos] = c_top_snow
 					end
 				end
-				if #snowdirt > 1 then
+				if #snowdirt > 0 then
 					lvm_used = true
 				end
 
+				--[[ Remove broken double plants caused by v6 weirdness.
+				v6 might break the bottom part of double plants because of how it works.
+				There are 3 possibilities:
+				1) Jungle: Top part is placed on top of a jungle tree or fern (=v6 jungle grass).
+					This is because the schematic might be placed even if some nodes of it
+					could not be placed because the destination was already occupied.
+					TODO: A better fix for this would be if schematics could abort placement
+					altogether if ANY of their nodes could not be placed.
+				2) Cavegen: Removes the bottom part, the upper part floats
+				3) Mudflow: Same as 2) ]]
+				local plants = minetest.find_nodes_in_area(emin, emax, "group:double_plant")
+				for n = 1, #plants do
+					local node = vm:get_node_at(plants[n])
+					local is_top = minetest.get_item_group(node.name, "double_plant") == 2
+					if is_top then
+						local p_pos = area:index(plants[n].x, plants[n].y-1, plants[n].z)
+						if p_pos then
+							node = vm:get_node_at({x=plants[n].x, y=plants[n].y-1, z=plants[n].z})
+							local is_bottom = minetest.get_item_group(node.name, "double_plant") == 1
+							if not is_bottom then
+								p_pos = area:index(plants[n].x, plants[n].y, plants[n].z)
+								data[p_pos] = c_air
+								lvm_used = true
+							end
+						end
+					end
+				end
+
 
 			-- Non-v6 mapgens:
 			-- Clear snowy grass blocks without snow above to ensure consistency.