Use spawn_batch in many_lights example

[BD103]

Objective

• The many_lights example uses a for-loop around commands.spawn.
• It is generally recommended to use spawn_batch instead to lazily spawn entities, because it doesn't massively grow the command queue.

Solution

• Use spawn_batch in many_lights example.

---

Discussion

• thread_rng is called for each light spawned. This is a simple thread-local Rc clone, so it should compile down to a copy and an increment + decrement instruction.
• I created golden_ration outside of the closure and moved it in. This should just be a copy and hopefully will get const-evaluated away. Would it be better to just move it into the closure itself?

Performance

Using spawn_batch seems to decrease time-to-first-Update by 0.1s: 1.3s to 1.2s.

Raw data and how it was collected.

Before:

• 2024-02-19T15:18:57.650987Z to 2024-02-19T15:18:58.912244Z : 1.3
• 2024-02-19T15:19:25.277135Z to 2024-02-19T15:19:26.542092Z : 1.3
• 2024-02-19T15:19:46.841460Z to 2024-02-19T15:19:48.137560Z : 1.3

After:

• 2024-02-19T15:17:05.749521Z to 2024-02-19T15:17:06.993221Z : 1.2
• 2024-02-19T15:17:38.153049Z to 2024-02-19T15:17:39.393760Z : 1.2
• 2024-02-19T15:18:10.691562Z to 2024-02-19T15:18:11.891430Z : 1.2

To time performance, I tracked the time from the first Startup logged message to the first Update logged message.

$ cargo run --release --example many_lights
Compiling bevy v0.13.0 (/Users/bdeep/dev/bevy/bevy)
    Finished release [optimized] target(s) in 1.54s
     Running `target/release/examples/many_lights`
# THIS TIME
2024-02-19T15:30:13.429609Z  INFO bevy_render::renderer: AdapterInfo { name: "Apple M1", vendor: 0, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "", backend: Metal }
2024-02-19T15:30:13.566856Z  INFO bevy_winit::system: Creating new window "many_lights" (0v1)
2024-02-19T15:30:13.592371Z  WARN many_lights: This is a stress test used to push Bevy to its limit and debug performance issues. It is not representative of an actual game. It must be run in release mode using --release or it will be very slow.
2024-02-19T15:30:13.592572Z  INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "MacOS 14.2.1 ", kernel: "23.2.0", cpu: "Apple M1", core_count: "8", memory: "16.0 GiB" }
# TO THIS TIME
2024-02-19T15:30:15.429900Z  INFO many_lights: Lights: 100000
2024-02-19T15:30:15.430139Z  INFO bevy diagnostic: fps        :    0.982693   (avg 43.026557)
2024-02-19T15:30:15.430157Z  INFO bevy diagnostic: frame_time : 1017.611750ms (avg 149.456476ms)
2024-02-19T15:30:15.430165Z  INFO bevy diagnostic: frame_count:   12.000000   (avg 6.000000)