Working with no GTK errors whatsoever, crashes on GL init.
Yea, that's definitely a crash if you don't return there, even on Apple (wtf?).
Make Radiant honor GDK_GL_LIBGL_PATH. This way Radiant and GtkGLExt can use the same instance of libGL.dylib. Failure to do this results in unpredictable and wrong behavior from GL.