[ptx] nona, gimp-2, TIFF_mask multilater tiff files

[Bruno Postle]

gimp-2 pre1 has just been released, and according to the release
notes it has "Support for multipage TIFFs [Andrey Kiselev]".

I thought I would try it with the nona TIFF_mask output.

The file loads but with a couple of problems:

-  All file offsets are ignored, each layer is fitted to the top
   left corner and the overall image is resized to the dimensions of
   the first layer.  This is not so bad, it just means that the
   image needs to be resized and the layers dragged to fit.

-  The masks are loaded as alpha-channels with no pixel information
   remaining in the fully transparent areas, so there is no way to
   blend the images by hand as there would be if the masks were
   loaded as layer masks.

-- 
Bruno