Query attachment render targets from Impl class. This allows us to eradicate the GetAttachmentRenderTarget methods. This improves potential performance, at the cost of exposing a Renderer-specific function at the API object level. BUG=angleproject:963 Change-Id: Ifc227b5f42e87bd4deb451d685618cf61fea39f1 Reviewed-on: https://chromium-review.googlesource.com/263491 Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>