Viewport focus is not updated when the mouse leaves the current window.

The viewport focus does not return false when the mouse leaves the screen and clicks somewhere else.

1. Download the attached project
2. Open the project
3. Play in stand alone window
4. Press the '~' key 3 times
5. Notice that the words lost focus appear (expected)
6. Move the mouse outside of the window and click
7. Move mouse back to the window and click
8. Notice that lost focus is never fired

Expected: Lost focus would fire when the mouse leaves the window and clicks somewhere else
Result: The viewport focus is not set to false when the mouse leaves the window

[BUG] GameViewportClient LostFocus and RecievedFocus fail to fire