D3D11: Downsample when copying to emulated 16-bit texture formats. BUG=angleproject:2313 Change-Id: Ic1e679164d82f3024bb6842c9af2716aa10feb76 Reviewed-on: https://chromium-review.googlesource.com/900042 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>