Fix mouse capture.

Fixes #15.

Perspex.Input/IPointerDevice.cs

@@ -11,9 +11,9 @@ namespace Perspex.Input
 
     public interface IPointerDevice : IInputDevice
     {
-        IInteractive Captured { get; }
+        IInputElement Captured { get; }
 
-        void Capture(IInteractive control);
+        void Capture(IInputElement control);
 
         Point GetPosition(IVisual relativeTo);
     }

Perspex.Input/MouseDevice.cs

@@ -30,7 +30,7 @@ namespace Perspex.Input
                 .Subscribe(this.ProcessRawEvent);
         }
 
-        public IInteractive Captured
+        public IInputElement Captured
         {
             get;
             protected set;
@@ -47,7 +47,7 @@ namespace Perspex.Input
             protected set;
         }
 
-        public virtual void Capture(IInteractive control)
+        public virtual void Capture(IInputElement control)
         {
             this.Captured = control;
         }
@@ -127,7 +127,7 @@ namespace Perspex.Input
 
         private void MouseDown(IMouseDevice device, uint timestamp, IInputElement root, Point p)
         {
-            IVisual hit = root.InputHitTest(p);
+            var hit = this.HitTest(root, p);
 
             if (hit != null)
             {
@@ -169,7 +169,7 @@ namespace Perspex.Input
 
         private void MouseUp(IMouseDevice device, IInputElement root, Point p)
         {
-            IVisual hit = root.InputHitTest(p);
+            var hit = this.HitTest(root, p);
 
             if (hit != null)
             {
@@ -190,7 +190,7 @@ namespace Perspex.Input
 
         private void MouseWheel(IMouseDevice device, IInputElement root, Point p,  Vector delta)
         {
-            IVisual hit = root.InputHitTest(p);
+            var hit = this.HitTest(root, p);
 
             if (hit != null)
             {
@@ -222,5 +222,10 @@ namespace Perspex.Input
                 (hit as IInteractive) ?? 
                 hit.GetSelfAndVisualAncestors().OfType<IInteractive>().FirstOrDefault();
         }
+
+        private IInputElement HitTest(IInputElement root, Point p)
+        {
+            return this.Captured ?? root.InputHitTest(p);
+        }
     }
 }

Windows/Perspex.Win32/Input/WindowsMouseDevice.cs

@@ -8,6 +8,7 @@ namespace Perspex.Win32.Input
 {
     using System;
     using Perspex.Input;
+    using Perspex.Interactivity;
     using Perspex.Win32.Interop;
 
     public class WindowsMouseDevice : MouseDevice
@@ -31,6 +32,20 @@ namespace Perspex.Win32.Input
             internal set { base.Position = value; }
         }
 
+        public override void Capture(IInputElement control)
+        {
+            base.Capture(control);
+
+            if (control != null)
+            {
+                UnmanagedMethods.SetCapture(this.CurrentWindow.Handle.Handle);
+            }
+            else
+            {
+                UnmanagedMethods.ReleaseCapture();
+            }
+        }
+
         protected override Point GetClientPosition()
         {
             UnmanagedMethods.POINT p;