Draft: network streaming driver

WiVRn stands for WIreless Virtual Reality n (n stands for nothing)

The driver relies on a standalone headset running wivrn application, the application multicasts an announce packet, monado driver listens for this packet and then initiates a connection.

When a connection is established: Monado driver creates a compositor target, backed by a pseudo-swapchain which encodes frames in h264/h265. The application sends tracking data regularly, which is stored on monado and served when pose, view or input are requested.

Application to be executed on headset is on https://github.com/Meumeu/WiVRn and tested mostly on Quest.

Note: As x264 is licensed under GPL2+ only, this merge request adds an XRT_ENABLE_GPL option (off by default) to accept this. Anyone who distributes monado binaries using this switch must comply with GPL for monado service.