crash when rawing opacity layer in release mode with the Qt backend

[ogoffart]

Can be reproduce by opening the item in the pinter queue in the printer demo using the viewer or the rust build, as long as slint is build in release mode.

==591737== Invalid read of size 8
==591737==    at 0x53EC854: QPen::style() const (in /usr/lib/libQt5Gui.so.5.15.3)
==591737==    by 0x53C6E96: ??? (in /usr/lib/libQt5Gui.so.5.15.3)
==591737==    by 0x52311CC: QTextLine::draw(QPainter*, QPointF const&, QTextLayout::FormatRange const*) const (in /usr/lib/libQt5Gui.so.5.15.3)
==591737==    by 0x53D0724: ??? (in /usr/lib/libQt5Gui.so.5.15.3)
==591737==    by 0x53BD70C: QPainter::drawText(QRectF const&, int, QString const&, QRectF*) (in /usr/lib/libQt5Gui.so.5.15.3)
==591737==    by 0x888F1C: __cpp_closure_14562761572348636077 (in /home/rust/sixtyfps/sixtyfps/target/release/slint-viewer)
==591737==    by 0x386564: <i_slint_backend_qt::qt_window::QtItemRenderer as i_slint_core::item_rendering::ItemRenderer>::draw_text (in /home/rust/sixtyfps/sixtyfps/target/release/slint-viewer)
==591737==    by 0x4BB4EA: i_slint_core::items::TextVTable::render (in /home/rust/sixtyfps/sixtyfps/target/release/slint-viewer)
==591737==    by 0x4CF62E: i_slint_core::item_tree::ItemVisitor_vtable_mod::ItemVisitorVTable::new::visit_item (in /home/rust/sixtyfps/sixtyfps/target/release/slint-viewer)
==591737==    by 0x6CBEF0: i_slint_core::item_tree::visit_item_tree::{{closure}} (in /home/rust/sixtyfps/sixtyfps/target/release/slint-viewer)
==591737==    by 0x69DB4B: slint_interpreter::dynamic_component::visit_children_item (in /home/rust/sixtyfps/sixtyfps/target/release/slint-viewer)
==591737==    by 0x390023: i_slint_core::properties::PropertyTracker<ChangeHandler>::evaluate_as_dependency_root (in /home/rust/sixtyfps/sixtyfps/target/release/slint-viewer)
==591737==  Address 0x244c8948c7894e0d is not stack'd, malloc'd or (recently) free'd

This maigt be because we swap the QPainter, but QPainter is not meant to be moved in memory because QPainterPrivate has a q_ptr pointing back to the QPainter.
Not sure why this doesn't cause any crash or valgrind warning in debug mode but only in release.

Also not setting the clip from QtItemRenderer::render_layer makes the crash disapear.

[ogoffart]

( I reported a qttypes::QPainter issue on the qttypes repository woboq/qmetaobject-rs#267 )