Fix remap_tiles_for_output() for -W16 (#57)

This commit fixes a bug where the tile data output was squished and
missing even columns when converting a SNES tileset with a `-W 16`
argument.

This commit also fixes a segfault when converting a snes tileset with
`-W 8 -H 16` (when _tile_width is 8 `(cells_per_tile_h - 2)` underflows).

src/Tiles.cpp

@@ -240,7 +240,7 @@ std::vector<Tile> Tileset::remap_tiles_for_output(const std::vector<Tile>& tiles
 
     for (unsigned i = 0; i < tiles.size(); ++i) {
       unsigned base_pos =
-        (((i / tiles_per_row) * cells_per_tile_v) * cells_per_row) + ((i % tiles_per_row) << (cells_per_tile_h - 2));
+        (((i / tiles_per_row) * cells_per_tile_v) * cells_per_row) + ((i % tiles_per_row) * cells_per_tile_h);
       const auto ct = tiles[i].crops(8, 8);
       for (unsigned cy = 0; cy < cells_per_tile_v; ++cy) {
         for (unsigned cx = 0; cx < cells_per_tile_h; ++cx) {