xrCreateSession assumes OpenGL context is current
xrCreateSession with an XrGraphicsBindingOpenGLXlibKHR results in client_gl_xlib_compositor_create() calling gladLoadGL() apparently without first checking if it needs to make the specified OpenGL context current (and switching back afterwards). If the specified OpenGL context is not current then glad_glGetString(GL_VERSION) returns NULL and GL_EXT_memory_object won't be found.
The spec states "Note that the OpenGL context given to the call xrCreateSession must not be bound in another thread when calling the functions: xrCreateSession, [...]. It may be bound in the thread calling those functions. The runtime must not access the context from any other function."
Note, the context may be bound (and clearly should be safely bindable by the runtime), but I read that as it doesn't have to be bound.
This is hit with current versions of osgXR (with FlightGear), which I'll probably add a workaround to for the time being.