Move the copySubBuffer extension over to the new mechanism.

src/mesa/drivers/dri/common/dri_util.h

@@ -73,6 +73,11 @@ typedef struct __DRIutilversionRec2    __DRIutilversion2;
  */
 extern __GLcontextModes *__driDriverInitScreen(__DRIscreenPrivate *psp);
 
+/**
+ * Extensions.
+ */
+extern const __DRIcopySubBufferExtension driCopySubBufferExtension;
+
 /**
  * Used by DRI_VALIDATE_DRAWABLE_INFO
  */
@@ -461,8 +466,12 @@ struct __DRIscreenPrivateRec {
     /**
      * Pointer back to the \c __DRIscreen that contains this structure.
      */
-
     __DRIscreen *psc;
+
+    /**
+     * Extensions provided by this driver.
+     */
+    const __DRIextension **extensions;
 };