Check to ensure the game is active before processing events (#196)

XNA/MonoGame captures all mouse and keyboard events, even when the window doesn't have focus.  This doesn't make a lot of sense (because it means you can click through other applications to the game window behind them), so this change ensures that the event engine hook checks to see if the game is active before firing any kind of event.

Protogame/Core/CoreGame.cs

@@ -216,6 +216,14 @@ public bool IsMouseVisible
             set { if (_hostGame != null) { _hostGame.IsMouseVisible = value; } }
         }
 
+        public bool IsActive
+        {
+            get
+            {
+                return _hostGame?.IsActive ?? false;
+            }
+        }
+
         /// <summary>
         /// Initializes an instance of a game in Protogame.  This constructor is always called
         /// as the base constructor to your game implementation.

Protogame/Core/ICoreGame.cs

@@ -24,6 +24,8 @@ public interface ICoreGame
 
         bool HasLoadedContent { get; }
 
+        bool IsActive { get; }
+
         void AssignHost(IHostGame hostGame);
 
         void EnableImmediateStartFromHost();

Protogame/Core/IHostGame.cs

@@ -28,5 +28,7 @@ public interface IHostGame
         SpriteBatch SplashScreenSpriteBatch { get; }
 
         Texture2D SplashScreenTexture { get; }
+
+        bool IsActive { get; }
     }
 }

Protogame/Events/EventEngineHook.cs

@@ -91,6 +91,11 @@ public void Render(IGameContext gameContext, IRenderContext renderContext)
         /// </param>
         public void Update(IGameContext gameContext, IUpdateContext updateContext)
         {
+            if (!gameContext.Game.IsActive)
+            {
+                return;
+            }
+
             UpdateKeyboard(gameContext);
             UpdateMouse(gameContext);