libtiff 4.x and custom tags

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What steps will reproduce the problem?
1. Add custom tags of specific data type using the "add_tags" function in 
libtiff_ctypes.py
2. Read or write file with these custom tags.
3. Get segmentation fault or assert(0)

What is the expected output? What do you see instead?
With libtiff 3.9 there are no segmentation faults or assertion errors or other 
crashes.

Please use labels and text to provide additional information.
I originally added the custom tag functionality to libtiff_ctypes.py. I was 
updating some of my environments that use pylibtiff with libtiff 4.0.3 and 
found that the tests at the bottom of libtiff_ctypes cause a segmentation fault 
that seems to be linked to the custom tag test. Also, when using custom tags 
there seems to be certain types that aren't supported in the new libtiff layout 
which results in an assert(0) error. One specific example is a tag with type 
"TIFF_SLONG" will be assigned an internal type of "TIFF_SETGET_SINT32", but the 
code to handle this type does not exist later in the library calls.

I think the "correct" decision is to update libtiff_ctypes to check the version 
of libtiff used and if 4.0+ use the new functions for adding custom tags. The 
problem with that is I don't think there is any documentation for creating 
custom tags in 4.0. I've asked the libtiff mailing list for any advice and I'll 
go from there.

Original issue reported on code.google.com by dho...@gmail.com on 17 Feb 2015 at 4:10