Change the validation to the consistent style for easy auto-generation
and make the context itself implement the entry points. This will more
easily allow us to sync state without passing the Context to the
Texture methods, or doing work in the entry point.

BUG=angleproject:1260
BUG=angleproject:747

Change-Id: I7ed6ec5418b7f51d9e59529267b14b76b87743fb
Reviewed-on: https://chromium-review.googlesource.com/319823
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>