#4 choppy paper advance on color print with black content

[Mark Roth]

If the driver is in color mode but we're only printing
black, the driver still advances the paper by only 64
pixels at a time. This causes it to advance 3 times
for each line of black, even if there is only black data.

Fix so that if there was no color data we can skip by
192 pixels at a time. That is, don't try and print a
color band if there is nothing to print.