D3D11: Fix 3D ReadPixels with PBOs. Pass the FBO attachment to the Buffer packing method so we can retrieve the layer of the attachment when doing an asynchronous readback. Also take advantage of the TextureHelper11 class to remove some redundant code. BUG=angleproject:1290 Change-Id: I26bb21a03e0ff7a42aab4eee75f3c3d12915f398 Reviewed-on: https://chromium-review.googlesource.com/324021 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>