Artifact mode improvements

[dshadoff]

Currently, artifact mode is set on the second half of a 2-pixel couplet (at 256 horizontal pix), and repeated in the following entry.
This means that:
a) the first active pixel of the line will need to be set to black.
b) the last pixel of the line only retains its color for a half-width (1 pix instead of 2).

A better approach would involve having a 2-pixel (or 1 artifacted pixel) look-behind and look-ahead buffers, and calculate based on that:
if prev pixel/next pixel is white and current is artifacted, make it a lighter color.
if prev pixel/next pixel together with current pixel results in a '00' combination crossing a pix boundary, make the articfacted pixel(s) half-width.