This encapsulates the workaround in the D3D renderer, and also
optimizes the workaround to only compute a new set of attachments
when there is a change to the state.

BUG=angleproject:930

Change-Id: Ibdc15078236e2d19b544fae8e65b7f2554f31844
Reviewed-on: https://chromium-review.googlesource.com/254102Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>