It appears that not only render targets, but also samplers that use
IOSurfaces require SwiftShader to perform synchronized draws,
otherwise it seems like using a shared IOSurface's CPU memory is
unsafe and can cause crashes in Layout Tests.

Bug chromium:846693

Change-Id: I0ce24700d34c657ac2447ceb2f6f837bfa3a9a58
Reviewed-on: https://swiftshader-review.googlesource.com/19288Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>