lavfi/delogo: don't recompute the same difference again and again

The top left hand corner pixel coordinates are already stored in
logo_x1 and logo_y1 so don't recompute each of them 6 times for every
iteration.

This is a simple code optimization, result is obviously the same. The
performance gain is small (about 2% in my tests) but still good to
have, and the new code is clearer.

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