Add zoomIn, zoomOut and zoomOut to Scalable2i

[navispatial]

Scalable base class:

  /// Zooms in by one.
  Scalable zoomIn() => Scalable(zoom: zoom + 1);

  /// Zooms out by one.
  /// 
  /// The minimum value for [zoom] of the returned scalable object is 0.
  Scalable zoomOut() => Scalable(zoom: math.max(zoom - 1, 0));

  /// Zooms to the [zoom] level (a positive number).
  Scalable zoomTo(covariant num zoom) => Scalable(zoom: zoom);

Scalable2i:

  @override
  Scalable2i zoomIn() => Scalable2i(
        zoom: zoom + 1,
        x: x << 1,
        y: y << 1,
      );

  @override
  Scalable2i zoomOut() => zoom == 0
      ? this
      : Scalable2i(
          zoom: zoom - 1,
          x: x >> 1,
          y: y >> 1,
        );

  @override
  Scalable2i zoomTo(int zoom) {
    assert(zoom >= 0, 'Zoom must be >= 0');
    if (this.zoom == zoom) {
      return this;
    }
    final shift = zoom - this.zoom;
    return shift > 0
        ? Scalable2i(
            zoom: zoom,
            x: x << shift,
            y: y << shift,
          )
        : Scalable2i(
            zoom: zoom,
            x: x >> shift.abs(),
            y: y >> shift.abs(),
          );
  }

[navispatial]

Implemented by beta 0.2.0.