The computation could copy more data than needed. Although benign
it could cause performance degredation compared to the D3D9 back
end.

Change-Id: I3cf1dc79085c33d44040fd55153e63a4e5e63cb1
Reviewed-on: https://chromium-review.googlesource.com/210640Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>