Nested Mir-on-Wayland should support windowed mode

[Conan-Kudo]

Summary

Mir-on-X11 opens up in a window while Mir-on-Wayland opens up fullscreen. This discrepancy is weird and makes Mir development a bit difficult on Wayland environments.

Details

Basically, Mir-on-Wayland should work the same way as Mir-on-X11. Furthermore, for the windowed mode, it should leverage xdg-decoration to use server-side decorations (SSDs) whenever possible. This could be accomplished by using libdecor or making your own implementation using existing decoration code.

Prior Art

KWin and Weston both implement windowed nested operation on Wayland and will automatically do it properly when run with no arguments (ie. kwin_wayland or weston). KWin even uses SSDs in this case.

[Saviq]

it should leverage xdg-decoration to use server-side decorations (SSDs) whenever possible

That's the annoying part, and part of why we didn't yet get to this - the X11 route avoids us this problem entirely…

[Conan-Kudo]

It should be possible to avoid the difficult parts with libdecor, I think?

[Saviq]

But that's an extra dependency, there's always a trade-off.