Software adapter should be requestable via env.var.

As QTBUG-78648 shows, sometimes we will want to use a software-based adapter / physical device. For APIs other than D3D and Vulkan this will not be applicable as we do not have the necessary level of support for enumerating adapters / physical devices.

To make this simple, QRhi has a flag called PreferSoftwareRenderer which picks the first software / CPU adapter / physical device. This may also be handy for testing (CI) purposes later on. As the public interface to QRhi is currently Qt Quick, we need to have a QSG_ environment variable that corresponds to this flag.