vbo: fix glPrimitiveRestartNV crash inside a display list

vbo: fix glPrimitiveRestartNV crash inside a display list

glPrimitiveRestartNV crashes when it is called during the compilation
of a display list.

There are two reasons:
- ctx->Driver.CurrentSavePrimitive is not set to the current primitive
- save_PrimitiveRestartNV() calls _save_Begin() which only sets an
  OpenGL error, instead of calling vbo_save_NotifyBegin().

This patch correctly calls vbo_save_NotifyBegin() but it detects
the current primitive mode by looking at the latest saved primitive.

Additional work by Brian Paul

Signed-off-by: Olivier Lauffenburger <o.lauffenburger@topsolid.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101464
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>