Make microblocks work for nodes with layered textures

[Caellian]

Currently, only topmost layer texture gets picked up and mapped to the side it's on on the original node.

This makes dirt with grass stairs use a stretched-out single-pixel high texture on the sides for instance.

This is a low-priority bug as there are more important things that should be done first though. For now, those nodes are just ignored during registration.

For a list of affected nodes, look at commented out nodes in #176.