use system cursor position for zoom when cursor is invisible (e.g. dragging).

Assets/uDesktopDuplication/Scripts/Monitor.cs

@@ -1,4 +1,5 @@
 using UnityEngine;
+using System.Runtime.InteropServices;
 
 namespace uDesktopDuplication
 {
@@ -157,6 +158,24 @@ public class Monitor
         get { return Lib.GetCursorY(id); }
     }
 
+    public int systemCursorX
+    { 
+        get 
+        { 
+            var p = Utility.GetCursorPos();
+            return p.x - left;
+        }
+    }
+
+    public int systemCursorY
+    { 
+        get 
+        { 
+            var p = Utility.GetCursorPos();
+            return p.y - top;
+        }
+    }
+
     public int cursorShapeWidth
     { 
         get { return Lib.GetCursorShapeWidth(id); }

Assets/uDesktopDuplication/Scripts/Utility.cs

@@ -0,0 +1,30 @@
+using UnityEngine;
+using System.Runtime.InteropServices;
+
+namespace uDesktopDuplication
+{
+
+public struct MousePoint
+{
+    public int x;
+    public int y;
+}
+
+public static class Utility
+{
+    [DllImport("user32.dll", SetLastError = true)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    public static extern bool GetCursorPos(out MousePoint point);
+
+    public static MousePoint GetCursorPos()
+    {
+        MousePoint p;
+        if (!GetCursorPos(out p)) {
+            p.x = -1;
+            p.y = -1;
+        }
+        return p;
+    }
+}
+
+}

Assets/uDesktopDuplication/Scripts/Utility.cs.meta

@@ -0,0 +1,12 @@
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+timeCreated: 1478934399
+licenseType: Pro
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+  defaultReferences: []
+  executionOrder: 0
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