Create ext-screencopy-v1 and ext-image-source-v1

This is a new screencopy protocol which improves upon wlr-screencopy-unstable-v1.

Supersedes: !39 (closed)

• Review
• 2 ACKs
  • wlroots (@emersion)
  • COSMIC (@Drakulix)
• 2 implementations
  • wlroots: wlroots/wlroots!3457
  • wayvnc: https://github.com/any1/wayvnc/tree/ext-screencopy-v1