[ptx] hugin/nona bug sampling image edges

[Bruno Postle]

I'm Trying to remap a full equirectangular image to equirectangular
output, but the edge pixels are being lost in both the hugin preview
and the nona output.

Here is the output using PTStitcher (source image has a pitch of -90
degrees):

   http://bugbear.blackfish.org.uk/~bruno/hugin/test-ptstitcher.jpg

The nona output from the same script has extra black areas (top
middle and top corners, otherwise it is identical):

   http://bugbear.blackfish.org.uk/~bruno/hugin/test-nona.jpg

If anyone is wondering why I need to do this, the idea is to take
this image:

   http://bugbear.blackfish.org.uk/~bruno/photos/t1-don-valley/panorama-1999-12-29.jpg

..and remap it to this fisheye view:

   http://bugbear.blackfish.org.uk/~bruno/hugin/panorama-1999-12-29-fisheye-small.jpg

Nona doesn't support fisheye output, but the gimp plugin can convert
from equirectangular to fisheye.

-- 
Bruno