[feature] explicit scene rectangle

[dai-shi]

Currently, the rectangle is determined from the size of the first scene, which is not very intuitive. It would be nice to have something explicit like excalideck printable rectangle. https://github.com/excalideck/excalideck/issues/7 This would require more sophisticated setting panel. excalideck does it well too.

[grant-davidson]

At the moment, it's worse than that because the size is determined by the size of the first scene at the time a second scene is added. After that (without deleting scenes), its not possible to change the size.