[ptx] stitcher -> nona

[Kai-Uwe Behrmann]

Hi,
the multilayer thing should be fixed in cvs. It was related to not
calculated alpha; due to an test? Pablo please see my comment. (Hope I did
not brake anything else.)

The other thing was related to negative offsets. They are not allowed so I
introduced two simple correction variables.

Attached is a screenshot of natively written out multilayerd nona-tiffs.
Very impressive. I had not the insider knowledge to do it as fast as You.

Thanks to Pablo.
Kai-Uwe