[idd] d3d12: signal reusable command slots

Signal a per-slot event after callbacks finish and the command slot has
returned to the free state. This removes the one millisecond polling delay
when candidate state becomes visible just before its slot is reusable.

idd/LGIdd/CD3D12CommandQueue.h

@@ -65,6 +65,7 @@ class CD3D12CommandSlot
     std::atomic<State>        m_state       = STATE_FREE;
     std::atomic<bool>         m_submitted   = false;
     HandleT<HANDLENullTraits> m_event;
+    HandleT<HANDLENullTraits> m_availableEvent;
     HANDLE                    m_waitHandle  = INVALID_HANDLE_VALUE;
     bool                      m_needsReset  = false;
     UINT64                    m_fenceTarget = 0;