function generateRotationMatrix(azimuth, elevation) {
    var forward = [
      -Math.sin(azimuth) * Math.cos(elevation), Math.sin(elevation),
      -Math.cos(azimuth) * Math.cos(elevation)
    ];
    var right = normalize(crossProduct([0, 1, 0], forward));
    var up = normalize(crossProduct(forward, right));
    return right.concat(up.concat(forward));
  }
Generate a col-major 3x3 Euler rotation matrix.
@param {Number} azimuth
@param {Number} elevation