Disable pausing during score screens in marathon mode

This is a proposed fix for #477 (closed) . Prevent pausing while the IGT timer isn't running.

In marathon mode you can abuse the in-game timer stopping between each level. You can pause the game when the timer wouldn't run without any time penalty. This fix prevents pausing during the score screen when the timer doesn't run, making it impossible to take a break between the levels. Pausing when a level is loading is already impossible.