add method to signal a main loop exit from the game app class

Blarg.GameFramework.SDL2/SDLLooper.cs

@@ -163,6 +163,12 @@ namespace Blarg.GameFramework
 			ReleaseSDL();
 		}
 
+		public override void Quit()
+		{
+			Logger.Info(LOG_TAG, "Quit signaled. Main loop will exit.");
+			_isQuitting = true;
+		}
+
 		private void MainLoop()
 		{
 			_isWindowActive = true;

Blarg.GameFramework/BaseLooper.cs

@@ -32,6 +32,7 @@ namespace Blarg.GameFramework
 		public bool IsRunningSlowly { get; protected set; }
 
 		public abstract void Run(IGameApp gameApp, IPlatformConfiguration config);
+		public abstract void Quit();
 
 		protected void OnAppGainFocus()
 		{

Blarg.GameFramework/ILooper.cs

@@ -27,6 +27,7 @@ namespace Blarg.GameFramework
 		bool IsRunningSlowly { get; }
 
 		void Run(IGameApp gameApp, IPlatformConfiguration config);
+		void Quit();
 	}
 }
 

Game.Core/GameApp.cs

@@ -55,6 +55,8 @@ namespace Game
 
 		public void OnUpdate(float delta)
 		{
+			if (Platform.Keyboard.IsPressed(Blarg.GameFramework.Input.Key.Escape))
+				Platform.Looper.Quit();
 		}
 
 		public void Dispose()