Sample WDDM driver

Writing a WDDM driver is very complex and I am not aware of any open-source examples that could be used as a reference. This is a problem for developers of virtualization toolkits, such as Qubes OS, who want to provide seamless integation of Windows guests into the host desktop environment. A sample driver that just rendered each window in software and then called a callback with the rendered buffer would be very useful for these developers.