Refactor Geometry Shader support to enable pass-through. This should be a refactoring change only. The new code is exercised in follow-up CLs. BUG=angleproject:754 Change-Id: I99285e1e7772cae467013102f25c911ebc9f54a9 Reviewed-on: https://chromium-review.googlesource.com/309153 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>