* You can now browse for a custom wallpaper image to use with your
levels. A platform-native file picker dialog is used (no WASM support)
* In the New/Edit Level Properties dialog, the Wallpaper drop-down
includes an option to browse for a custom map.
* When editing an existing level: the wallpaper takes effect immediately
in your level once the file is picked. For NEW levels, the wallpaper
will appear once the "Continue" button is pressed.
* All common image types supported: png, jpeg, gif.
* The wallpaper is embedded in the level using the filepath
"assets/wallpapers/custom.b64img" as a Base64-encoded blob of the
image data.
* The `doodad show` command will list the names and sizes of files
embedded in levels. `doodad show --attachment <name>` will get an
attachment and print it to the console window.
* To extract a wallpaper image from a level:
`doodad show -a assets/wallpapers/custom.b64img | base64 -d > out.png`
