Fix off by one error compared to original Acidwarp 4.10.

The XMax/xmax, YMax/ymax and Colormax/colormax values were
misunderstood in the original Linux port (c48e818). They are
actually size values, and not maximums like the names imply.
They came from a table in graf.c. For the commonly used mode,
they are: XMax = 320, YMax = 200 and Colormax = 256.

Setting colormax to 255 inappropriately limited colours to
1 to 254 via modulus (colormax - 1). This made the
horizontal seam going right from the centre more visible
in integer mode. A double width stripe going right from
the centre still remains in screen 13 in integer mode, like
in original Acidwarp 4.10. That is because angle goes from
0 to 255, but colours go from 1 to 255.

Variables are renamed to avoid further confusion. This will
make comparing to original code more difficult, but
generate_image() has alredy been moved to another file,
with whitespace changes.