RFE: Video loopback support

It's very common for content creators (e.g. streamers, YouTube creators, etc.) to take a video source and apply effects and redirect it into another application (like OBS Studio). Today, this is often done by using the v4l2loopback kernel module to create V4L2 sources that can be manipulated by applications.

However, v4l2loopback is not in the kernel. Moreover, it does not even really make sense that it needs to be in the kernel for this to work. It'd be great if PipeWire provided a way to weave in things into a video source and export a video source that applications can consume.

This is essentially an identical use-case to the audio loopback support requested in #543 (closed), only aimed at video sources instead.