This enables usage of Nvidia GPU on a system which has both Nvidia and non-Nvidia graphics. A new port is needed since nvidia-driver by itself expects to be the sole provider of GLX, conflicting with other drivers. Creating an nvidia-headless-driver slave port is less invasive than rewriting parts of nvidia-driver to handle all system configurations correctly.
VirtualGL is used by nvidia-hybrid-graphics nvrun-vgl command to move rendered frames from Nvidia GPU to the user's X11 display.
The most critical aspect of this review is that we must ensure that the changes to nvidia-driver port are correct, that is they must not change the behavior of that port when used alone.