Add track design screenshot command to CLI

[iamthad]

The CLI currently has a screenshot command to generate a screenshot of an entire park.

It would be very nice to also be able to quickly generate a screenshot of a particular track design.

This could be useful for track design authors, as well as players with many track designs who wanted to browse through them outside the game.

I imagine this could either be implemented as:

• An additional mode for the existing screenshot command, handled based on file extension or format
• Or a new command, perhaps placed under a track subcommand for future expansion, e.g. track screenshot