Query attachment render targets from Impl class. *re-land with fix for D3D9* 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: Iee9f985ddaed668df0c622228004b348eb4d2ea8 Reviewed-on: https://chromium-review.googlesource.com/269006 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>