Get Pi Camera to appear as a USB webcam by configuring it as a Linux gadget

A lot of info here: 

https://publiclab.org/questions/warren/12-15-2017/how-can-we-set-up-a-raspberry-pi-camera-as-an-ethernet-usb-device-to-avoid-using-wifi

This is hinted at in these docs:

https://learn.adafruit.com/turning-your-raspberry-pi-zero-into-a-usb-gadget/ethernet-gadget

http://www.circuitbasics.com/raspberry-pi-zero-ethernet-gadget/