Warn when "misaligned" schematics are created

The underlying issue is the size detection worldedit.allocate does
and can't be fixed or worked around in a simple way.

worldedit_commands/init.lua

@@ -1097,6 +1097,23 @@ minetest.register_chatcommand("/restore", {
 	end),
 })
 
+local function detect_misaligned_schematic(name, pos1, pos2)
+	pos1, pos2 = worldedit.sort_pos(pos1, pos2)
+	-- Check that allocate/save can position the schematic correctly
+	-- The expected behaviour is that the (0,0,0) corner of the schematic stays
+	-- sat pos1, this only works when the minimum position is actually present
+	-- in the schematic.
+	local node = minetest.get_node(pos1)
+	local have_node_at_origin = node.name ~= "air" and node.name ~= "ignore"
+	if not have_node_at_origin then
+		worldedit.player_notify(name,
+			"Warning: The schematic contains excessive free space and WILL be "..
+			"misaligned when allocated or loaded. To avoid this, shrink your "..
+			"area to cover exactly the nodes to be saved."
+		)
+	end
+end
+
 minetest.register_chatcommand("/save", {
 	params = "<file>",
 	description = "Save the current WorldEdit region to \"(world folder)/schems/<file>.we\"",
@@ -1112,6 +1129,7 @@ minetest.register_chatcommand("/save", {
 		end
 		local result, count = worldedit.serialize(worldedit.pos1[name],
 				worldedit.pos2[name])
+		detect_misaligned_schematic(name, worldedit.pos1[name], worldedit.pos2[name])
 
 		local path = minetest.get_worldpath() .. "/schems"
 		-- Create directory if it does not already exist