Merge car meshes in bevy_city and improve loading screen

[IceSentry]

Objective

• In the asset pack used for bevy_city the tires of all the cars are separate meshes from the car body. In some of the cars even the doors are also separate meshes.
• Since we don't animate these elements of the car we can trivially combine all the meshes of each car scene into one single car mesh. This helps a lot to keep performance manageable and eventually increase the scale of bevy_city.

Solution

• Wait for the assets to load and once they are loaded loop over all the car scenes and combine their meshes. This is mainly possible because the entire asset pack uses a single texture so we don't need a separate material for each part of the mesh.
• To do this I also refactored a bit how each step happens when starting the app. I also made sure that each step is reflected on the loading screen.
• I also added a bit more docs

Testing

• I ran bevy_city and confirmed that the tires are where they should be relative to the mesh and that performance was better. I went from 90fps to 112fps on my machine

---

Showcase

bevy_city_process_assets.mp4

[ChristopherBiscardi]

changes seem good to me.

The ui is... really, really small to the point of being unusable in this demo. Did I miss a setting or control?

[ChristopherBiscardi]

should mention what performance issues specifically this was meant to optimize, otherwise this will become cargo-culted forward without context in the future.

[IceSentry]

Oh, yeah, I didn't really think about that kind of impact. I'll try to rephrase that.

[IceSentry]

I updated the comment, please let me know what you think. I think I gave enough details as to why this is done.

[ChristopherBiscardi]

it's great, thanks!