Add 4-cell mosaic graphics.

Add 4-cell mosaic graphics.

These follow the same approach as 6-cell graphics: the smoothing
algorithm is disabled and there's no margin between character cells.
This is necessary to ensure that "upblock" and "dnblock" harmonise
with "lfblock" and "rtblock", the latter two being unified with 6-cell
graphics and the former not.

I've filled in the rest of the 4-cell characters on the basis that an
alternate history in which Teletext used 4-cell rather than 6-cell
graphics is perfectly plausible and Bedstead may as well support it.