Fix single buffer copies. 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/210902 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nicolas Capens <capn@chromium.org>