int im_line(image, x1, y1, x2, y2, pelval) IMAGE *image; int x1, x2, y1, y2, pelval;
draws a line in the image held by image. The start of the line is at point
(x1,y1) and the end is at (x2,y2). The intensity of the drawn line is pelval.
Input image should be one byte unsigned char. The function overwrites any
data in the file, so take care when applying it.