Add: allow loading heightmaps from command-line (#11870)

If you want to load a file from tar, you have to give the file
inside the tar in order for it to work:

<tar-file>/<dir-in-tar>/<file>.png

src/landscape.h

@@ -139,6 +139,6 @@ void DoClearSquare(TileIndex tile);
 void RunTileLoop();
 
 void InitializeLandscape();
-void GenerateLandscape(byte mode);
+bool GenerateLandscape(byte mode);
 
 #endif /* LANDSCAPE_H */