Added a glXCreateContextVendorDUMMY extension function to GLX_dummy to test
using a vendor-provided extension function to create a context. We can't use
glXCreateContextAttribsARB for that now, because it has a dispatch stub in
libGLX itself.
glXCreateContextVendorDUMMY takes the same arguments as
glXCreateContextAttribsARB, so it can also serve as an example for implementing
a dispatch stub for glXCreateContextAttribsARB in a real vendor library.