// a) so we don't have to load the image here to know its dimensions
// b) so that the resulting 'tiles' entry is more robust,
// ie. enlarging the image won't break the tile id mapping
// a) so we don't have to load the image here to know its dimensions
// b) so that the resulting 'tiles' entry is more robust,
// ie. enlarging the image won't break the tile id mapping