Use clamp cast for stencil mask in all versions. dEQP only covers this behaviour in ES2.0 tests and they fail when running with an ES3.1 context. BUG=angleproject:3425 Change-Id: I339b83901434da92ae6dd6273460b0725644cdf5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1602177 Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>