[ptx] hugin interpolator problems

[Rich]

while trying out different interpolators, i noticed some problems and 
weird things.

1. sinc 256 produces artifacts in the sky

this seems similar to problem enblend used to have
http://enblend.sourceforge.net/banding.htm
though artifacts are different here.

a quick picture fragment with artifacts : sinc_256_screen.jpg

2. nearest neighbour produces black areas - is this supposed to be so ?

3. poly3 vs sinc1024 - different resulting image size (with same parameters)

actually it's image data that is simetrically smaller. sinc 1024 is sort 
of cropped. sinc 256 also is smaller, though not as much as 1024. other 
interpolators also produce slightly differently sized images.

is this ok ?

transformed images are available here :
evf.lv/stuff/images.tar.bz2
(big archive, has tiffs)

original project (for reproduction) here :
evf.lv/stuff/project.tar.bz2
-- 
  Rich