[ptx] feathered seams in nona

[Pablo d'Angelo]

Hi!

I've just added a simple seaming to hugin. Its a quick hack, that was easy
to add. it works well for large seams too. actually its a mixture between
the credibility pixel approach and the classical seaming.
Arbitrary seam widths should work, no black artefacts will appear.

Technical info:
it uses the following formula to calculate weight factors for a single point 
p:

d = dist_center(img_i(p)) - mindist;
if d < seam width
    weight_i = d
else
    weight_i = 0

then, the final pixel value is simply a weighted sum:
value =  sum_i( img_i * weight_i) / sum_i(weight_i)

was easy to add, and works quite well for the areas where images meet (
img.dist_center similar), but not so nice for the upper/lower edges.

Ideally I should use the real from the seam, but then I'll have to compute
the seams explicitely, and can't only use the distance maps.

Haven't added the seaming support for multiple tiff output yet, because the
weight factors I'm using are not alpha values.

ciao
  Pablo
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