[ptx] Nearest-neighbour bug in nona

[John Bäckstrand]

Nona gives different results than PTStitcher for nearest neighbour 
sometimes, sometimes it seems fine though:

http://sandos.ath.cx:8000/pics/near/

I think its at higher resolutions it starts showing this bug. PTStitcher 
doesnt show this behaviour at any "approximate" setting, afaik. I dont 
really use nearest neighbour, I like sinc 1024 better, but I dont know, 
someone might? :)
 
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John Bäckstrand