generateVertexPositions method 
    
    
    
  Implementation
  @override
void generateVertexPositions(Vector3List positions, Uint16List indices) {
  var i = 0;
  // Top
  for (var x = 0; x <= _segments; ++x) {
    final u = x / _segments;
    positions[i++] = Vector3(_topRadius * math.cos(u * math.pi * 2.0),
        _height * 0.5, _topRadius * math.sin(u * math.pi * 2.0));
  }
  // Bottom
  for (var x = 0; x <= _segments; ++x) {
    final u = x / _segments;
    positions[i++] = Vector3(_bottomRadius * math.cos(u * math.pi * 2.0),
        _height * -0.5, _bottomRadius * math.sin(u * math.pi * 2.0));
  }
  // Top cap
  for (var x = 0; x < _segments; ++x) {
    final u = x / _segments;
    positions[i++] = Vector3(_topRadius * math.cos(u * math.pi * 2.0),
        _height * 0.5, _topRadius * math.sin(u * math.pi * 2.0));
  }
  // Bottom cap
  for (var x = 0; x < _segments; ++x) {
    final u = x / _segments;
    positions[i++] = Vector3(_bottomRadius * math.cos(u * math.pi * 2.0),
        _height * -0.5, _bottomRadius * math.sin(u * math.pi * 2.0));
  }
}