`AtmosphereEnvironmentMapLight` and `GeneratedEnvironmentMapLight` are extremely slow

[beicause]

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:

[dylansechet]

The reason seems to be that the environment maps are refiltered every frame. The performance issue with the basic GeneratedEnvironmentMapLight could probably be fixed by implementing the re-generation control mentioned in #20212. I'll try to open a PR for that.