Rationalise mosaic graphics implementation

Rationalise mosaic graphics implementation

The "separated graphics" flag is now in the "flags" field, rather than
in the character data, and the bits specifying which cells are painted
are now contiguous.  Also the MOS flag is now called MOS6, to go with
MOS4.  This makes the specification of the ZVBI code points more
complex, but cleans up the Unicode ones, which I think is appropriate.

On the way, I found that 'palt' was being applied to 4-cell mosaic
graphics and fixed that.