Support ext-image-capture-source-v1 and ext-image-copy-capture-v1

[Ferdi265]

This completely flew under the radar for me. These two protocols were just merged into wayland-protocols and if implemented in wlroots would potentially allow window screenshare in wlroots compositors.

This has less priority than #16 since xdg-desktop-portal-wlr will likely expose window capture as well, but this protocol likely has better selection ergonomics on some desktops (allowing to specify an output on the commandline).