[Geotiff] Add support for TIFFOpenStream

[Fred Rothganger]

Dear GeoTIFF Folks,

    I would like to use the C++ stream code in libtiff (tiffio.hxx, 
tif_stream.cxx) for file i/o.  libxtiffio supports every method of 
opening a TIF other than this.  At least two solutions come to mind:

    1) One of the developers comes up with a nice way to add the 
TIFFStreamOpen() functions.

    2) Make _XTIFFInitialize() public.  Since this is the only thing 
special about the XTIFFOpen() functions, it is just as simple to allow 
the client code to call it, and then use whatever TIFFOpen it wishes.

-- Fred