Implement classifying image enums in utilities and plumb the image
type check to Uniform objects as well. This functionality will be used
to add API support for images.

BUG=angleproject:1442

Change-Id: Idd2ee5ebacd38290aeeace1177165c740bc42c7e
Reviewed-on: https://chromium-review.googlesource.com/441585
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>