Automatic Skybox -> EnvironmentMapLight generation

[JMS55]

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

glTF-IBL-Sampler is the current recommended way to generate EnviornmentMapLight (IBL) for Skyboxes. However, it's very difficult to build the codebase, and does not work for realtime procedurally generated skyboxes, only static offline skyboxes.

What solution would you like?

Implement Fast Filtering of Reflection Probes.

Each frame, Bevy should take entities with Skybox a component and no EnvironmentMapLight component, create an EnvironmentMapLight component with empty textures, and then dispatch some compute shaders to fill in the textures.

Open Questions

To control the IBL generation, we should either add a marker component like AutomaticSkyboxLight, or add a boolean field to the Skybox component. I'm not sure which would be better. I'm leaning towards the boolean.

Additional context

godotengine/godot#36588