feat: add optional background blur via image crate

Gaussian blur applied at texture load time when `background-blur` is
set in the [appearance] section of moongreet.toml. Blur runs once,
result is shared across monitors.

Cargo.toml

@@ -15,6 +15,7 @@ gio = "0.22"
 toml = "0.8"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+image = { version = "0.25", default-features = false, features = ["jpeg", "png"] }
 log = "0.4"
 systemd-journal-logger = "2.2"