tileUv method
Maps a within-tile coordinate (u, v, each in [0, 1] with (0, 0) at
the tile's top-left) to an atlas UV for tile index.
Implementation
Vector2 tileUv(int index, double u, double v) {
final bounds = tileBounds(index);
return Vector2(
bounds.x + (bounds.z - bounds.x) * u,
bounds.y + (bounds.w - bounds.y) * v,
);
}