Minetest_game: Add barrier block

[Fixer-007]

Not inventing wheel here, it is "transparent solid block used to create impenetrable boundaries", used by server admins to create boundaries for various purposes.

[An0n3m0us]

I've created it, but it is TNT-able. Not sure how to prevent that.

[LoneWolfHT]

You can return true/false (I forget which) with on_blast in your node def to prevent TNT from destroying it

[An0n3m0us]

Thanks; it worked.

0.06% alpha. I've also made the inventory image white (using the cloud texture).

Which tool should be used to break the barrier?

[An0n3m0us]

How about an admin pick or something of the type?

[LoneWolfHT]

An admin pick like the superpick in Minetest would be nice. But I think for now it would be better to just set barrier blocks to be able to dig themselves

[paramat]

I am not convinced this is a good idea for MTG. Such barriers are usually a specialist usage by servers, and servers likely need differing characteristics according to their need.

[paramat]

See PR (closed) for more discussion.