Refactor FBO related entry points. 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>