Known bugs:
* The Pixel format in the Grid has DX and DY attributes and
it wreaks havoc on collision detection in Play Mode when you
come straight from the editor. Reloading the map from disk to
play is OK cuz it lacks these attrs.
First pass at a level storage format to save and restore maps.
To save a map: press F12. It takes a screenshot PNG into the
screenshots/ folder and outputs a map JSON in the working directory.
To restore a map: "go run cmd/doodle/main.go map.json"
