Improve Glass Protections for Stairways and Bridges

[BrainSlugs83]

When the outside filler block is not "air", we should put a cylindrical tube around the bridge and/or stairway to protect them (keep them filled with air, and keep lava and water out, etc.).

Right now the bridge has a rectangular enclosure, but it's open on the bottom, so if the bridge is wood, and it's over lava, it may eventually still burn up, etc.

The tube block should be configurable as well (maybe you do want it to be wood, even if the outside block is lava -- it will give your bridges a longer survival life time, etc...)