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>