Allow exporting levels to PNG level preview file

[Matriks404]

[Matriks404]

It would be cool if the resulting preview file instead of the static background would have gradient instead (such as cyan->black from bottom to top), the idea comes from a Super Mario Bros. 2 1-1 wallpaper.

The final result would probably look best if freeroam level type was used (where you can scroll both vertically and horizontally, which is not something doable in the original Super Mario Bros. 2, but I am planning to add that level type).