*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/269006Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>