358.16 Unable to use full OpenGL with EGL

One of the items in the changelog from 355.06 was “Added experimental full OpenGL support to EGL”.
This doesn’t appear to work correctly in 358.16.

I’m using the example EGL & OpenGL code from here: http://bobthegnome.blogspot.ca/2014/09/using-egl-with-gtk.html
It compiles successfully with:

gcc -g -Wall gtk-egl.c -o gtk-egl `pkg-config --cflags --libs gtk+-3.0 gdk-x11-3.0` -lEGL -lGL

It runs without error, but the window is just black with no triangle.

If I try glGetString(GL_VERSION), it returns NULL.

Is there something extra that needs to be done to enable the experimental EGL_OPENGL_API support, or is it just not at a fully working stage yet?