This subtest verifies that glXGetProcAddress() works for the various
classifications of "GetProcAddress-able" functions:
- GLX 1.4 functions with dispatchers implemented directly by the API
library
- GLX 1.4 extension functions with dispatchers implemented by a vendor
loaded by the API library
- Core OpenGL functions with static dispatch stubs in libGLdispatch
- Core OpenGL functions with dynamic dispatch stubs generated by
libGLdispatch
This also tests some assertions about these dispatch stubs; namely that
the core GL dispatch stubs are no-ops if no context is current.
