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>
lavfi/delogo: remember left and right samples when interpolating

The left and right samples are the same for the whole line, so store
their values and don't recompute them for every iteration of "y".

This simple optimization results in a speed improvement between 15%
and 20% in my tests (depending on the logo geometry.)

Result is obviously the same.

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