[stella] PF register write timing

[bob.montgomery at thomson.com]

Hi,

> BTW, there are cycle timings that can result in
>bleed-over at an almost sub-pixel level into the
>playfield block.

As far as I know, and as far as I've seen, color register changes
happen immediately, so a playfield pixel could be two colors.  I don't
think you can write half of a PF pixel to the screen, though.

-bob