fix: prevent edge darkening on GPU-blurred wallpaper (v0.5.3)

GskBlurNode samples pixels outside texture bounds as transparent,
causing visible darkening at wallpaper edges. Fix renders the texture
with 3x-sigma padding before blur, then clips back to original size.
Symmetric fix with moonset v0.7.1.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "moongreet"
-version = "0.5.2"
+version = "0.5.3"
 edition = "2024"
 description = "A greetd greeter for Wayland with GTK4 and Layer Shell"
 license = "MIT"