[ptx] autopano-sift 2.0 and 48bit tiff bug

[Katherine Donaldson]

Hello, autopano-sift (2.0) doesn't seem to properly work with 48bit 
tiffs.  I'm running autopano-win-gui if that matters.

It seems to read them in fine, they look fine in the preview pane, the 
keypoint generation appears to work (it generates non-empty keypoint 
files anyhow) but after it appears to have matched all the keypoints it 
puts up an error that says construction of the bond ball has failed.

If I run with the exact same tifs but converted to 24bit, it works 
fine.  So I assume there is something wrong with the calculation when 
the images are 48bit.

-Katherine