nixos/sway: set default wallpaper

[Synthetica9]

Motivation for this change

cc @primeos

See also: https://github.com/Synthetica9/nixpkgs/pull/new/sway-default-wallpaper

The wallpaper shows up in the test, but I don't know if this is what you had in mind.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[primeos]

The wallpaper shows up in the test, but I don't know if this is what you had in mind.

Yeah, unfortunately not quite, sorry about that... I've drafted #122995 with what I had in mind.

[stale]

I marked this as stale due to inactivity. → More info

[Synthetica9]

Still relevant I'd say

[primeos]

We still have to decide if we want to go with this PR or #122995 (which was a bit of an unfortunate situation). IIRC I prefer my approach (duh) but that is certainly open for discussion (plus I have to take another look at it). However, I'd be nice if we could wait until the weekend (for my replies/opinion, the discussion can of course already continue).

@Synthetica9 I assume you prefer your approach, right?

[Synthetica9]

Might be nice to install the Nix-themed or sway-themed wallpaper depending on whether we're running on NixOS or not, but I have no strong preference. Feel free to merge whichever one and close the other.

[primeos]

Might be nice to install the Nix-themed or sway-themed wallpaper depending on whether we're running on NixOS or not, but I have no strong preference.

Yeah, by default I'd prefer the upstream default from Sway though as I don't really like downstream patches/changes when avoidable. However, I don't know how other community members feel about this and unfortunately we don't have any formal guidelines (at least AFAIK).

Anyway, the implementation is fine but I don't really like this approach for the following reasons:

• It adds an additional option to the NixOS module and IMO defaultWallpaper doesn't seem relevant enough (but who knows, maybe many users would like such an option to more easily refer to dynamic Nix store paths)
• It hardcodes fill and always applies the wallpaper to all outputs (e.g. I use different wallpapers per monitor but that might be an uncommon approach - then again it e.g. makes a lot of sense if the monitors have different resolutions).
  • Note: This isn't an issue for the default but it makes the option less flexible/useful.
• It links to the wallpaper via /etc/sway/wallpaper which doesn't feel right to me.
• It might unexpectedly override user-defined wallpapers if they include /etc/sway/config.d/* (it's of course fixable by setting defaultWallpaper to null but I'd like to avoid that).

Feel free to merge whichever one and close the other.

@Synthetica9 so if you're fine with it I'd prefer to go with #122995. It's not perfect either but it seems good enough to me for now.