[REG: 6.8.3 -> 6.8.4] transparent windows no longer work with QSG_RHI_BACKEND=opengl on Windows

The attached project uses a transparent window with a drop shadow while using the opengl RHI backend (as specified in main.cpp). This works as expected on Qt 6.8.3:

but the transparency doesn't work on Qt 6.8.4 on Windows (it still works on Linux, have not tested on other platforms):

Changing to the default RHI backend makes this work (but we require the opengl backend for other reasons).