- Nadir and Zenith in 360x360 panos.

- Allow users to provide their own blending masks. Have an option to write out
  the mask that enblend calculates to a TIFF file.

- Support multilayer TIFFs.

- Stitching discrepancy avoidance in mask creation.

- Make assemble smarter.

- For win32, recompile libtiff with TIF_PLATFORM_CONSOLE
- remove tiff warning handler = null
- add errno checks around libtiff calls in vigra_impex