Background atoms

[Zac8668]

Add a background atom layer where solids would be able to be placed. This would be useful to add some background walls to your house, cool mining mechanics, etc.
What's up for discussion is if solids would need or not a background atom to be able to support themselves, this would allow for more realistic behavior but maybe it would be a bit annoying. If a solid is in the world but without a background atom behind it, we could make it behave like a powder with 1.0 inertial resistance, so it would only fall down and not to the sides.

This means we would also need a toggle on the tool precision mode, between the background and normal layer.

A cool mechanic that this can add is to make rare ores to be most commonly found on the background layer, that way your 2d vision can't help you, you would have to dig the normal layer to uncover the background layer to find the ores.

Not sure how this would impact performance, but I think not much, as the background layer would not need to update, just render itself.