Like textures, renderbuffers don't become real objects until they are
bound.  If the renderbuffer is never bound before being attached to a
framebuffer, it would generate an INVALID_OPERATION.

Fixes:
 * conformance/textures/tex-input-validation.html
 * conformance/renderbuffers/renderbuffer-initialization.html

BUG=angleproject:886

Change-Id: I639b9e2d9ae694dce29f20ed5f95077ae02d471e
Reviewed-on: https://chromium-review.googlesource.com/273533Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>