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/324021Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>