Stop using FramebufferAttachment::Target. Target includes the binding (DEPTH/STENCIL/COLOR), which is not useful for many operations. Simplify this to just passing the mip/layer. This allows us to stop using this internal struct in other classes. BUG=angleproject:1635 Change-Id: Ic5a11781bf45fe7835437fa1e363c190b876d453 Reviewed-on: https://chromium-review.googlesource.com/469152 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>