Feature: Variable GUI scale.

GUI scale is now variable from 100% to 500%, and no longer restricted to
powers-of-2.

src/gfx_func.h

@@ -79,8 +79,7 @@ void ChangeGameSpeed(bool enable_fast_forward);
 void DrawMouseCursor();
 void ScreenSizeChanged();
 void GameSizeChanged();
-void UpdateGUIZoom();
-bool AdjustGUIZoom();
+bool AdjustGUIZoom(bool automatic);
 void UndrawMouseCursor();
 
 /** Size of the buffer used for drawing strings. */