[ptx] difference preview mode

[Pablo d'Angelo]

Hi!

Well, I've been inconsequent and added a new feature...

Did you ever wonder how well your images really fitted? I used to do this
with multilayer files in gimp and difference blending. however this was a
lot of work, and completely out of the usual workflow.

I've added several blending modes to the preview panel:

1. Normal, simply copies the images over eachother
2. Difference, displays the absolute difference between images.
   This gives a good visual feedback withc parts to not match
   correctly yet.
3. seam, places a seam through the images. No soft blending yet though.

The only drawback with difference is that currently the images are blended
after eachother, if an uneven number of perfectly fitting images is put
together then the image itself is shown. This defeats the purpose of seeing
the differences easily.

However, for panos that are not to irregular the blending, the blending mode
is a good indicator where additional control point should go. and to see if
the lens is properly calibrated.

ciao
  Pablo