* This is a bit of a hack. Mesa seems to return a function pointer for
functions it recognises but doesn't support. So e.g. if you query for
glBindFramebuffer you'll get the implementation, if you query for
glBindFramebufferEXT then you get a stub that errors out.
* So in this case we ensure that any EXT/ARB aliases only set their
function pointer if we don't have the core function already. This will
fix some cases but obviously doesn't help if the core function is the
error-stub and the extension is supported...