Add NewGRF feature: Extra station name strings

These are used when all the default names have been used up,
instead of "Town Station #NNN".

src/gfxinit.cpp

@@ -214,6 +214,9 @@ static void LoadSpriteTables()
 	/* Initialize the unicode to sprite mapping table */
 	InitializeUnicodeGlyphMap();
 
+	extern uint _extra_station_names_used;
+	_extra_station_names_used = 0;
+
 	/*
 	 * Load the base and extra NewGRF with OTTD required graphics as first NewGRF.
 	 * However, we do not want it to show up in the list of used NewGRFs,