More complicated 3d scene example

[JMS55]

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

When developing new rendering features, we need test scenes. Bevy's current 3d examples are generally too simple, often exercising only one specific feature. Something like bistro is too heavy for quick iteration, and more complicated to setup (not a simple cargo run --example).

What solution would you like?

Add a new 3d example scene with medium complexity.

Important requirements:

• Static meshes, using normal maps and materials
• Dynamic meshes, that move around the scene in a set pattern
  • Should be able to pause their movement with a hotkey
• Transparent meshes
• Alpha-masked meshes
• Large meshes that block other meshes (for depth related testing) (e.g., a pillar)
• Point lights and directional lights
• Controllable camera
• Not too large/demanding- shouldn't be bigger than a room or two, and quick to load/run
• Developer options - toggle for debug visualizations, turning specific features on or off, logs, etc

A good starting point would be to build on example/tools/scene_viewer.rs.