[ptx] Bug: cannot load greyscale images

[Pablo d'Angelo]

Michal schrieb:
> The latest CVS version of hugin doesn't like greyscale images. It prints
> this error while loading image:
> 
> 06:46:00 PM: Error during image reading:  Precondition violation!
> importImage(): number of bands (color channels) in file and destination
> image differ. (../../src/foreign/vigra/impex.hxx:119)
> 
> It was possible to load these images some time ago. But white was
> displayed as red.

I have just commited a fix for this, now they should also be gray and not
red. If it doesn't work, please send me of of the images.

ciao
  Pablo