avfilter/delogo: Set default band to 1

The original interpolation algorithm behaved poorly on the borders and
did not even guarantee continuity at the borders. For this reason, a
second interpolation/blending pass was required on the borders to make
them seamless.

However, since the interpolation algorithm was improved in June 2013,
the border issues no longer exist. The new algorithm does guarantee
continuity at the borders, making the second pass useless. A larger
band always increases the cumulated interpolation error. In most cases
it also increases the average interpolation error, even though the
samples in the band are only partially interpolated.

For this reason I would like to get rid of the "band" parameter. As a
first step, let's change its default value from 4 to 1 and document it
as deprecated.

I have benchmarked this change on a combination of input sources and
realistic logo areas. Lowering the band value from 4 to 1 resulted in
8 to 39 % less interpolation error per frame (or 1 to 34 % less
interpolation error per luma sample.)

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

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