imageinterlace() turns the
interlace bit on or off. If interlace is 1 the image will be
interlaced, and if interlace is 0 the interlace bit is turned
off.
If the interlace bit is set and the image is used as a
JPEG image, the image is created as a progressive JPEG.
This function returns whether the interlace bit is set
for the image.