libkwineffects: Set format on the main OffscreenQuickView
When showing the window RHI will realise the window has no underlying platform window and create a temporary offscreen surface, different from the offscreen surface we'll eventually be rendering into. This will use the format of the main window.
If this offscreen surface format does not match the format of the context we can fail in makeCurrent calls.
On my machine I no longer get RHI warnings and crashes. Some UI like the kscreen OSD work perfectly, but I still have a different issue with fullscreen effects. David Redondo claims fullscreen effects work for him on Qt 6.4.