But wayland compositors should bypass compositing for fullscreen frames. There should be no overhead at all... at least if the client uses the wl_fullscreen_shell protocol extension, if I'm not mistaken.
Yes of course. For performance the same levels should be reachable with both approaches.
I just explained why Mir needs explicit support for this. A Wayland compositor could do
this without any support in the Wayland protocol (however support in the protocol would
be preferable (and may exist) for having compatibility between different compositors).
screencasting in display server? shouldn't this be in compositor? both wayland and mir already treat clients like sinks if i understand correctly, which would also mean you could enable screencasting in any compositor simply by using infrastructure they work on
I'm not sure about Mir but Wayland calls for the compositor to be the display server. That was one of the big motivating factors behind Wayland: to have the display server to be transform aware of the windows so no unneeded hops between the thing doing the transforms and the thing serving the final image.