Only currently Renderbuffers and Textures can be bound with images.
This makes the relationship explicit, and also ensures that an image
sibling can determine if it's been initialized or not, which is
important for the robust resource init extension with Textures and
EGLImages.

BUG=angleproject:1635

Change-Id: Ie05319cfbfda50f1497cab3f0b9ca64e1bff8955
Reviewed-on: https://chromium-review.googlesource.com/469154Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>