Add support for letterboxing

## What problem does this solve or what need does it fill?

Cutting off segments of the game screen using black bars (a letterbox) is a common technique for both cinematic cutscenes and for supporting unusual aspect ratios, particularly for pixel art.

## What solution would you like?

Expose a simple setting that sets the letterboxing mode, probably to Window or cameras.

## What alternative(s) have you considered?

Add an example demonstrating how to set this up manually, probably using render targets.

## Additional context

Discussed [on Discord](https://discordapp.com/channels/691052431525675048/1271949921708146770/1282845399669018675) in the context of pixel art games. Prompted by the closure of #14153.