Use the framebuffer object's stencil attachment.

Bug 27460431

Change-Id: Ic030624cc34b31519882e54497457a1b92eaac45
Reviewed-on: https://swiftshader-review.googlesource.com/4981
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>