SOSetTargets should explicitly NULL any buffers it doesn't set
Fetch and serialise hidden stream-out buffer counters. Closes #105

* Essentially when streamout'ing to a buffer a hidden counter is saved with
  it that contains the number of primitives written.
* With this change we issue queries over each buffer's duration as a
  stream-out target so we have those counters available when necessary.
* At the start of frame capture, we save out the current value of all those
  queries, and use them if that buffer is used for a DrawAuto() without
  being streamout'd to again in the captured frame.