This actually adjusts the palette values used by the CTerm shadow
palette.  This allows the palette to work properly without the
weird offset thing.

Unfortunately, this means a cterm_reset() will not use the new
values, and since term.c doesn't know when cterm_reset() has been
called, we can't fix it afterward anymore without being very
invasive and double-checking after every cterm_write() call.

This is fine for the indended audience however (ie: Atari ST
emulation) since they don't have a reset sequence.