lavfi/delogo: option show shouldn't affect band

lavfi/delogo: option show shouldn't affect band

Options "show" and "band" are unrelated and should thus be
independent. However, setting "show" to 1 currently resets "band" to
its default value of 4. While this is documented, this still
surprising and confusing IMHO.

Change this behavior and make "show" and "band" independent from each
other. Update the documentation accordingly.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>