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.
For users of earliest Optimus GPUs (unsupported by 440.x drivers) there is FLAVOR=legacy variant of x11/nvidia-hybrid-graphics.
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.