Define an interim 2D-to-3D mapping data format

[kpreid]

We have many blocks whose voxels are defined by loading a 2D image and applying a function to the image’s pixels (space_from_image()). We should replace those functions with data files which can be stored next to the images. This will hopefully reduce compilation time and make future migrations to better formats easier.

Alternatively, we can just go direct to storing the blocks in a voxel format (preferably an All is Cubes native one), but that's blocked on having adequate voxel editing tools.