lavfi/delogo: Fix sign extension issue

Coverity complains about a possible sign extension issue in
apply_delogo(). While it is extremely unlikely to happen, it is easy
to fix so let's just do that. Using unsigned variables even makes the
binary code smaller.

Fixes Coverity CID 1046439.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavfi/delogo: Document where parameter show=1 draws its rectangle

It is not completely obvious where delogo's parameter show=1 draws its
rectangle. Document it so that the user can more easily tune the
parameters.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>