Bind renderbuffers as soon as they are generated. 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/273533 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>