Render-to-texture widgets in the QRhi-based world (QQuickWidget, QOpenGLWidget)

Not for Qt 6.0.

What happens to render-to-texture widgets (like QQuickWidget and QOpenGLWidget) and the backingstore compositor? In line with the goal of removing direct OpenGL usage, this is heavily affected as well, but not yet clear what we want to do instead. (or if we want to do anything)

For 6.0-6.3 QQuickWidget is tied to OpenGL as per QTBUG-78638.

Qt 6.4 is getting a new QRhi-based widget composition solution in QtGui.
https://codereview.qt-project.org/c/qt/qtbase/+/379812
This then enables QQuickWidget to break free from the OpenGL-only limitation.
https://codereview.qt-project.org/c/qt/qtdeclarative/+/380411