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/heightmap.h

@@ -22,7 +22,7 @@ enum HeightmapRotation {
 };
 
 bool GetHeightmapDimensions(DetailedFileType dft, const char *filename, uint *x, uint *y);
-void LoadHeightmap(DetailedFileType dft, const char *filename);
+bool LoadHeightmap(DetailedFileType dft, const char *filename);
 void FlatEmptyWorld(byte tile_height);
 void FixSlopes();