In order to create the set of available extensions, we have to know if

In order to create the set of available extensions, we have to know if
direct-rendering is possible on a given screen.  It is not possible to
determine this by just looking at driDisplay.private in the display
structure.  However, it is possible to determine this by looking at
driScreen.private in the screen structure.  This field is only non-NULL if a
DRI driver has been successfully loaded and bootstrapped.  Once we get to
that point, we know that direct-rendering is at least theoretically
possible.

This fixes bugzilla #2875.