Default secondary tilesets use the wrong tiles

[GriffinRichards]

When you create a new tileset there's an option to have it filled with a checkerboard pattern. The new tiles image has two different tiles for this purpose. However when filling the metatiles we assume it's a primary tileset, so new secondary tilesets don't use the new tiles for the checkerboard pattern (which can create some unexpected-looking new tilesets, or use invalid tiles):