Add screenshot support

[JimmyCushnie]

Marble Marcher generates absolutely beautiful images, and it would be nice if we could easily capture those. Screenshots should:

• not render the UI
• render the game at a configurable resolution - for example, I may be running the game in 1080p, but want to take an 8k screenshot.

[WAUthethird]

It's now been added in the Community Edition: https://github.com/WAUthethird/Marble-Marcher-Community-Edition

You can change the resolution in assets/config.txt.