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/263491Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>