Actually dispose graphics resources

src/Game.cs

@@ -134,9 +134,12 @@ namespace MoonWorks
 
                     Draw(timestep, alpha);
 
+                }
+
 				GraphicsDevice.SubmitDestroyCommandBuffer();
             }
-            }
+
+			GraphicsDevice.Dispose();
         }
 
         private void HandleSDLEvents()

src/Graphics/GraphicsDevice.cs

@@ -101,8 +101,9 @@ namespace MoonWorks.Graphics
                 {
                     lock (resources)
                     {
-                        foreach (var resource in resources)
+						for (var i = resources.Count - 1; i >= 0; i--)
 						{
+							var resource = resources[i];
 							if (resource.TryGetTarget(out var target))
                             {
                                 target.Dispose();
@@ -111,6 +112,7 @@ namespace MoonWorks.Graphics
                         resources.Clear();
                     }
 
+					SubmitDestroyCommandBuffer();
                     Refresh.Refresh_DestroyDevice(Handle);
                 }
 
@@ -118,7 +120,6 @@ namespace MoonWorks.Graphics
             }
         }
 
-        // TODO: override finalizer only if 'Dispose(bool disposing)' has code to free unmanaged resources
         ~GraphicsDevice()
         {
             // Do not change this code. Put cleanup code in 'Dispose(bool disposing)' method