[REG 5.15.2 -> 6.0.0] OpenGL backend flickers during window resize on macOS

When I set `QSG_RHI_BACKEND=opengl` and run a Qt Quick app built with Qt 6.0.0 on macOS 11.1, it exhibits flickering when resizing the window. The flickering appears to be worse if the window is resized more slowly.

To reproduce, you can use any Qt Quick Controls application. I used the "Qt Quick Controls - Gallery" application.

1. Open the "Qt Quick Controls - Gallery" example application
2. Add `qputenv("QSG_RHI_BACKEND", "opengl");` to the top of main() in gallery.cpp.
3. Run the application
4. Resize the window by dragging slowly from one of the edges. The slower you drag, the more it seems to flicker.

Expected result:
Window contents scale gracefully without any rendering artifacts

Actual result:
Window contents flicker as the window is resized