vbo: disable the GL_ARB_draw_elements_base_vertex rebase path

This was introduced with commit 92d7ed8a20.
It causes rendering of stray polygons (with sw rendering at least) when
running the OGL Distilled / Picking demo (click on an object).

This needs additional debugging to fix/restore.

Found one suspect thing: in _tnl_draw_prims() there's some mixed signed/
unsigned arithmetic/comparing at line 422 that may be incorrect.

src/mesa/vbo/vbo_rebase.c

@@ -127,7 +127,10 @@ void vbo_rebase_prims( GLcontext *ctx,
       _mesa_printf("%s %d..%d\n", __FUNCTION__, min_index, max_index);
 
 
-   if (ib && ctx->Extensions.ARB_draw_elements_base_vertex) {
+   /* XXX this path is disabled for now.
+    * There's rendering corruption in some apps when it's enabled.
+    */
+   if (0 && ib && ctx->Extensions.ARB_draw_elements_base_vertex) {
       /* If we can just tell the hardware or the TNL to interpret our
        * indices with a different base, do so.
        */