avfilter/delogo: Fix show option when band is small

The code assumed that the outermost interpolated pixels were always in
the fuzzy area defined by the band option. However if the band value
is small, there may be no fuzzy area on a given plane. In that case,
option show did not work, no rectangle was drawn (or only on the luma
plane, depending on the band value and chroma plane subsampling
factors.)

Fix the problem by not making any assumption on where the outermost
interpolated pixels will be.

The new code was verified to produce the same result as the original
code when the band value is not small.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
