Swap completion suspended while QQuickItem hidden

This only affects the new viz display compositor mode.

Our render "loop" currently relies on triggering an update with QQuickItem::update and actually performing the update with QSGNode::updateRenderNode. The delay between these two steps can be arbitrarily long. In particular, when the item is hidden, then the second call only happens when the item next becomes visible.

This means, for example, that tst_qquickwebengineviewgraphics fails in viz mode because it operates with a hidden view. It works without viz though, so there's probably something wrong with our use of the QtQuick scene graph in viz mode.