qt6: Prevent qtquick settings from creating opengl context

The platform theme creates an opengl context to determine whether the graphics driver is broken. However, it does for all applications, including the ones that don't use OpenGL at all. For example, Konsole or Dolphin.

With Dolphin, it's even worse because it can create multiple kioworkers and each one of them will attempt to create an OpenGL context too.

This also slightly reduces the private memory usage. For example, for Konsole, it drops by 10MiB. For Dolphin itself, the memory usage drops by about 15MiB, but it doesn't account kioworkers. The shared memory usage drops by 100MiB for all cases.

While 10-15MiB savings may not be impressive at first, if you consider the number of apps that don't use OpenGL, the savings can still be significant.

OpenGL context creation is a pretty heavy process too, even with my beefy maching, it's takes about 20-30 ms to create one.

If the graphics driver breaks, the most important thing is that the user should be able to reboot the computer. In order to guarantee that, the opengl context creation step has been moved to the logout greeter itself.