Add support for generating 8-cell mosaic graphics

Add support for generating 8-cell mosaic graphics

This follows the pattern of the 6-cell and 4-cell versions, but without
separated mode because Unicode doesn't have that.  This required making
the bitmap data into an unsigned char array to allow for using all eight
bits.

The cell boundaries are chosen so that they line up with existing 6-cell
and 4-cell boundaries.  So we have row heights of [3,2,2,3], which is
maybe less pretty than [3,2,3,2], but is more in keeping with the
existing mosaics.  Indeed, it means that the existing 4-cell and 6-cell
contiguous characters could be expressed in 8-cell terms if I wanted to.