Fix save dialog centering

This issue exists because centerAllSurfaces() is called SavedGameState before surfaces in subclasses (in this case SaveState) have been created.  At the default resolution, this is not a problem, but when game assets are shown at 'x2' resolution, the save dialog is shoved off to the lower right corner of the screen.

This pull request fixes it by calling centerAllSurfaces() in the SavedGameState subclasses instead of in SavedGameState itself.