stand: lua: convert orb to a new format that allows per-graphic shift

Previously, we had one shift that had to work for both the ASCII version
and the image that we may draw if we have a framebuffer to play with.
That makes it more difficult to get the positioning just right, so split
them out into their own tables for convenience.

While we're here, stop clipping off the bottom of the orb when the
autoboot sequence is interrupted. There's plenty of space for it to
move up a hair, so let's do that..

Reviewed by: imp, tsoome
Differential Revision: https://reviews.freebsd.org/D50801