Add a Sprite3D demo

[chocola-mint]

There's no demo that uses Sprite3D or AnimatedSprite3D.

I was working on a rendering optimization PR involving SpriteBase3D and wanted a more realistic use case to test for edge cases, (particularly when multiple Sprite3Ds change their colors and get destroyed) but realized that there's no demo here that uses Sprite3D or AnimatedSprite3D.

The most common use case for Sprite3D would be either a Paper Mario-like or the "HD-2D" artstyle. A Paper Mario-like would likely be the better idea considering Sprite3D's current feature set. (Notably this means no need for normal maps, reducing the project's complexity) Though if Sprite3D ever gets official normal map support, an HD-2D artstyle might be better.