Move DM choices to "Video" in the menu option. Added two values to tune normal viewing distance and how much to lower the DM (to avoid conflict with normal tiles). Clean code for SuperElevation.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@1551 a9f3cfff-6c41-7446-b0ca-052bd761e6eb