These routines were pretty much duplicated between D3D9 and D3D11.
Since I was going to have to rewrite them again for Vulkan, I
figured it would be best to move them into a common location and
clean them up a bit.

BUG=angleproject:1319

Change-Id: I15d39b052daf3e1020dbd0880f01ae84f3686a0a
Reviewed-on: https://chromium-review.googlesource.com/349630Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>