UI/Qt: Present macOS frames with QRhiWidget

Use QRhiWidget for the Qt web content view on macOS and render the
current IOSurface-backed shared image into the widget's Metal render
target. This keeps normal presentation on the GPU instead of painting a
QImage wrapper over the shared bitmap.

Force Qt Widgets' RHI backing store to Metal before QApplication is
created so QRhiWidget can obtain the top-level QRhi even when the native
window is created before the web content widget enters the hierarchy.
Other platforms keep the existing QWidget and QPainter path.