Release wallpaper pixel memory after image changes

Store decoded pixels and RGB conversion scratch in anonymous mappings so dropping an image releases its pages. Reclaim decoder scratch on the worker thread after each load.

Verify PNG, JPEG and WebP pixels against the existing decoder and cover decode limits and corrupt files. Add an isolated release memory regression for repeated 4K wallpaper loads.

compositor/Cargo.toml

@@ -22,6 +22,7 @@ jiff.workspace = true
 notify = "8.2.0"
 zbus = "5.19.0"
 image = { version = "0.25.10", default-features = false, features = ["png", "jpeg", "webp"] }
+memmap2 = "0.9.11"
 smithay.workspace = true
 serde.workspace = true
 serde_json.workspace = true