blockworld -- sloped and corner blocks

[chrisforbes]

We kept putting these off in EN1. Let's get them right from the start this time.

[chrisforbes]

There's a couple headaches here:

• Inverse corner blocks have 7 faces, not 6, so we can't just take the existing faces and map them down.
• Raycast changes a little
• We /really/ want the faces to be submeshes in the framing mesh if we do this, to keep everything organised

[chrisforbes]

• Airtightness changes

[chrisforbes]

Thinking on atmo impact:

• Normal blocks are one node in the atmo topology.
• A sloped / corner / invcorner block is two nodes, if its sloped face is present and airtight. Otherwise, it's just one node.
• An airtight surface on a triangular face doesn't seal an adjacent full block's surface.

[chrisforbes]

Expected proportion of two-node blocks is <5%. This may drive storage decisions.