See the very last line of this page: http://esotericsoftware.com/spine-c: "The current setup does not support clipping." So far everything else I've tried using with my spine-c implementation is working. I don't know what version of spine-c I am using, but the source files say "Spine Runtimes Software License v2.5"
Edit: I appear to be using spine-c 3.6.
Also, I am very excited about the generic c++ runtime! Thank you for developing that, and also for keeping it in c++ 3 standard, I do the same thing with my c++ code. None of that fancy c++ 11 stuff. But I have spent so much time developing my own version that I'm not quite ready to move from spine-c to spine-c++ yet. When I have some spare time and a compelling reason I will definitely dive deep into it!