Currently we are about 2x as slow on this micro-benchmark in D3D11.
Preliminary testing suggests using CopySubresourceRegion gives
improved performance across the board, but testing could disprove
this hypothesis.

Modified from a sample from bajones.

BUG=angle:705

Change-Id: Iedf36a4e7b9b9bbed308302fd2bf3912acacbd2c
Reviewed-on: https://chromium-review.googlesource.com/216272Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>