In D3D11, the device doesn't allow overlapped buffer copies unless
the D3D11_FEATURE_DATA_D3D11_OPTIONS::CopyWithOverlap feature is
enabled. This isn't technically available until D3D11.1.

Get around the restriction by using a different storage buffer as
the source of the copy.

Fixes funcional.buffer.copy.single_buffer.* in dEQP.

BUG=angle:715

Change-Id: I03fea8a1c6180a29e07e05022e3454524e320803
Reviewed-on: https://chromium-review.googlesource.com/210902Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>