multiwindow manual test does not work with D3D12 backend

multiwindow_threaded works as expected when run with -D (i.e. using the D3D12 backend)

multiwindow on the other hand ends up in a device loss and opening the 2nd window makes the rendering freeze. This is the case when one QRhi has multiple QRhiSwapChains associated with it because the same QRhi is used to render to multiple windows sequentially from the same thread. This isn't something Qt Quick exercises (the Qt Quick basic/threaded render loop model is more like multiwindow_threaded, i.e. a dedicated QRhi per window and so per swapchain), so it does not occur much in practice, but should be fixed nonetheless.