title: How do I obtain the numeric device ID of a cl::Device?
body: <p>The OpenCL C++ bindings has the <code>cl::Device</code> class wrapping a <code>_cl_device_id *</code>. The pointed-to type is opaque to us.</p>
  <p>How can one obtain, given a <code>cl::Device</code>, its numeric ID (within its context)? The base-class' <code>operator()</code> only gives me the wrapped <code>_cl_device_id *</code>, not a number.</p>