Refactor validation of FramebufferRenderbuffer. Consolidate a lot of duplicated code into helper methods. Also move GetTargetTexture to the context class. Change-Id: I8e3990a5e3c1976cb533941d0a0946466accec30 Reviewed-on: https://chromium-review.googlesource.com/181556 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>