Add stepdown for currently supported state levels

src/NTwain/Triplets/DATParent.cs

@@ -22,6 +22,7 @@ namespace NTwain.Triplets
       STS rc;
       if ((rc = DoIt(MSG.OPENDSM, ref hwnd)) == STS.SUCCESS)
       {
+        Session._hwnd = hwnd;
         Session.State = STATE.S3;
         // determine memory mgmt routines used
         if ((((DG)Session._appIdentityLegacy.SupportedGroups) & DG.DSM2) == DG.DSM2)
@@ -46,8 +47,9 @@ namespace NTwain.Triplets
       STS rc;
       if ((rc = DoIt(MSG.CLOSEDSM, ref hwnd)) == STS.SUCCESS)
       {
-        Session.State = STATE.S2;
+        Session._hwnd = IntPtr.Zero;
         Session._entryPoint = default;
+        Session.State = STATE.S2;
       }
       return rc;
     }

src/NTwain/TwainSession.cs

@@ -6,7 +6,7 @@ using TWAINWorkingGroup;
 
 namespace NTwain
 {
-  // this file contains initialization-related things.
+  // this file contains initialization/cleanup things.
 
   public partial class TwainSession
   {
@@ -83,5 +83,22 @@ namespace NTwain
       DGImage = new DGImage(this);
       DGAudio = new DGAudio(this);
     }
+
+    internal IntPtr _hwnd;
+
+    /// <summary>
+    /// Tries to bring the TWAIN session down to some state.
+    /// </summary>
+    /// <param name="targetState"></param>
+    /// <returns>The final state.</returns>
+    public STATE TryStepdown(STATE targetState)
+    {
+      if (State > targetState)
+      {
+        // shouldn't care about handle when closing really
+        DGControl.Parent.CloseDSM(ref _hwnd);
+      }
+      return State;
+    }
   }
 }