computeKeyLevel method
Implementation
void computeKeyLevel(int level, Envelope itemEnv) {
double quadSize = math.pow(2.0, level) as double;
pt.x = (itemEnv.getMinX() / quadSize).floorToDouble() * quadSize;
pt.y = (itemEnv.getMinY() / quadSize).floorToDouble() * quadSize;
env.init(pt.x, pt.x + quadSize, pt.y, pt.y + quadSize);
}