Refactor separate sampler and image validations into unified opaque type handling. This paves way for adding atomic counter, another new opaque type. BUG=angleproject:1729 Change-Id: I201d28e31c84534db43e656d518650e378bab76c Reviewed-on: https://chromium-review.googlesource.com/493618Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>
