Markus and I started working on a GTK widget for embedding NeoEngine rendering into. We're actually doing it in gtkmm rather than GTK+, because NeoEngine is written in C++ and developers are more likely to prefer to write their apps in C++. Plus it is almost sickeningly simple to write it in C++ vs writing it in C.
I've been mostly busy re-organizing the code into a library, and dealing with auto* hackery today. I made a simple test that renders a triangle in a GTK window and posted a screenshot.