Codechange: Make TileIndex a "strong" typedef to give it a distinct type.

This is accomplished by changing it to a single member struct with the
appropriate operator overloads to make it all work with not too much
source modifications.

src/map.cpp

@@ -84,7 +84,7 @@ TileIndex TileAdd(TileIndex tile, TileIndexDiff add,
 		char buf[512];
 
 		seprintf(buf, lastof(buf), "TILE_ADD(%s) when adding 0x%.4X and 0x%.4X failed",
-			exp, tile, add);
+			exp, (uint32)tile, add);
 #if !defined(_MSC_VER)
 		fprintf(stderr, "%s:%d %s\n", file, line, buf);
 #else