Use attachment binding points for dynamic PS key. Because our output signature is only dependent on the arrangment of the attachments, not the attachment type, use the output layout key for now. If we need to, we could store both, in the future. BUG=angle:705 Change-Id: I3b99954d30b91a4741fdd6f48f8ffcf88c0bea7a Reviewed-on: https://chromium-review.googlesource.com/215846 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>