Change #6684: Cutting point overrides for music base sets

This improves bad looping of title screen song from Windows TTD, and fixes
a long silence at the end of "Can't get there from here" from Windows TTD.

(svn r25213) -Fix [FS#5537]: clarify on which tiles IsDesertTile and IsSnowTile work, i.e. the ones without infrastructure or buildings
-Feature: introduce GetTerrainType which allows one to get that information for tiles with buildings and infrastructure as well