`AtmosphereEnvironmentMapLight` and `GeneratedEnvironmentMapLight` are extremely slow

`AtmosphereEnvironmentMapLight` and `GeneratedEnvironmentMapLight` are extremely slow even on desktop:
```
SystemInfo { os: "Linux (Arch Linux)", kernel: "7.0.10-arch1-1", cpu: "AMD Ryzen 7 4800U with Radeon Graphics", core_count: "8", memory: "15.0 GiB" }
```

```
cargo r -p bevy_city -r -- --size 5
```
Without `AtmosphereEnvironmentMapLight` 58 fps:

With `AtmosphereEnvironmentMapLight` 13 fps:

```
cargo r --example reflection_probes -r
```
With `EnvironmentMapLight` 59 fps:

With `GeneratedEnvironmentMapLight` 17 fps:

Start by running `cargo r -p bevy_city -r -- --size 5` and `cargo r --example reflection_probes -r`, comparing the reported frame rates with and without `AtmosphereEnvironmentMapLight` and `GeneratedEnvironmentMapLight`. Trace the environment-map lighting entry points and measure where the frame time is spent. Done means identifying and addressing the slowdown while preserving the lighting behavior and improving performance toward the reported `EnvironmentMapLight` results.

Written by the indexing model from the issue text.