function QuadtreeNode(tilingScheme, parent, level, x, y) {
  this.tilingScheme = tilingScheme;
  this.parent = parent;
  this.level = level;
  this.x = x;
  this.y = y;
  this.extent = tilingScheme.tileXYToRectangle(x, y, level);

  this.rectangles = [];
  this._sw = undefined;
  this._se = undefined;
  this._nw = undefined;
  this._ne = undefined;
}
Computes a bit mask indicating which of a tile's four children exist.
If a child's bit is set, a tile is available for that child.  If it is cleared,
the tile is not available.  The bit values are as follows:
<table>
    <tr><th>Bit Position</th><th>Bit Value</th><th>Child Tile</th></tr>
    <tr><td>0</td><td>1</td><td>Southwest</td></tr>
    <tr><td>1</td><td>2</td><td>Southeast</td></tr>
    <tr><td>2</td><td>4</td><td>Northwest</td></tr>
    <tr><td>3</td><td>8</td><td>Northeast</td></tr>
</table>

@param {number} level The level of the parent tile.
@param {number} x The X coordinate of the parent tile.
@param {number} y The Y coordinate of the parent tile.
@return {number} The bit mask indicating child availability.