Undocumented behavior for QGraphicsItem: opacity == 0 has the same effect as isVisible() == false

For a QGraphicsItem, having opacity close to 0 has the same effect as having isVisible == false. Mouse events will go straight through it, QGraphicsScene::items(...) won't return it, etc.

This important behavior is not documented in QGraphicsItem::setOpacity() (or other opacity methods), unlike QGraphicsItem::setVisible().