Implement XDG-Decoration Protocol

[PolyMeilex]

I'm not sure how to check this:

Creating an xdg_toplevel_decoration from an xdg_toplevel which has a buffer attached or committed is a client error, 
and any attempts by a client to attach or manipulate a buffer prior 
to the first xdg_toplevel_decoration.configure event must also be treated as errors.

[i509VCB]

Off topic:

I do wonder, would having the wayland-rs's post_error take an Into<u32> reduce the amount of as u32 casts

[DemiMarie]

Would it be possible to also implement server-decoration?

[i509VCB]

Poly, my guess for ensuring no existing buffer is attached or commited is get the underlying state of the toplevel's wl_surface and check if some fields on SurfaceData attached to the wl_surface are None or Some?

[PolyMeilex]

Would it be possible to also implement server-decoration?

For sure, that's my plan, but for another PR, as xdg decoration is based on xdg toplevels, and kde's server decoration is based on wl-surfaces.

[elinorbgr]

I'm not sure how to check this: ...

This part cannot be checked in Smithay given the helper as you provide it, so it'll have to be delegated to either the compositor itself or some future higher-level abstraction.