Remove screenConfigs from __DRIscreen.

The screenConfigs field of __DRIscreen points back to the containing
__GLXscreenConfigs struct.  This is a serious abstraction violation; it
assumes that the loader is libGL and that there *is* a __GLXscreenConfigs
type in the loader.

Using the containerOf macro, we can get from the __DRIscreen pointer to
the containing __GLXscreenConfigs struct, at a place in the stack
where the above is a valid assumption.  Besides, the __DRI* structs shouldn't
hold state other than the private pointer.

include/GL/internal/dri_interface.h

@@ -87,7 +87,8 @@ typedef const char * (* PFNGLXGETDRIVERCONFIGPROC) (const char *driverName);
  * \c __DRIinterfaceMethods::getProcAddress.  This function is used to enable
  * a GLX extension on the specified screen.
  */
-typedef void (* PFNGLXSCRENABLEEXTENSIONPROC) ( void *psc, const char * name );
+typedef void (* PFNGLXSCRENABLEEXTENSIONPROC) (__DRIscreen *screen,
+					       const char *name);
 /*@}*/
 
 
@@ -299,16 +300,6 @@ struct __DRIscreenRec {
      */
     int (*getMSC)( void *screenPrivate, int64_t *msc );
 
-    /**
-     * Opaque pointer that points back to the containing 
-     * \c __GLXscreenConfigs.  This data structure is shared with DRI drivers
-     * but \c __GLXscreenConfigs is not. However, they are needed by some GLX
-     * functions called by DRI drivers.
-     *
-     * \since Internal API version 20030813.
-     */
-    void *screenConfigs;
-
     /**
      * Functions associated with MESA_allocate_memory.
      *