Add interfaces for overriding texture images with driver specific 'offsets'.

To be used by AIGLX for GLX_EXT_texture_from_pixmap without several
additional data copies.

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

@@ -189,6 +189,12 @@ struct __DriverAPIRec {
     /*@}*/
     void (*CopySubBuffer)(__DRIdrawablePrivate *driDrawPriv,
 			  int x, int y, int w, int h);
+
+    /**
+     * See corresponding field in \c __DRIscreenRec.
+     */
+    void (*setTexOffset)(__DRIcontext *pDRICtx, GLint texname,
+			 unsigned long long offset, GLint depth, GLuint pitch);
 };