qt quick: XWayland black screen with Nvidia proprietary driver

I'm not sure if this is a driver bug or Qt bug, but you can follow the steps below in order to make the application usable:

• Set environment variable QSG_NO_STENCIL_BUFFER, as for some reason stencil buffer is not granted. This is needed with Qt 5.15, but not with Qt 5.11.

For custom composition:

• Set requested stencil buffer size to 0 in compositor_x11_uisurface.cpp. This is needed with Qt 5.15, but not with Qt 5.11.
• Disable single buffer swap behavior setting in compositor_x11_uisurface.cpp. This is needed in all cases.