Allow creating `Image` from `Color` for use in `EnvironmentMapLight`

#18207 deprecated `AmbientLight` in favor of `EnvironmentMapLight` but uses an awkward constructor that accepts a reference to the `Image` assets collection in addition to a color.

We should instead have something like `Image::from_color` or `Image::cubemap_from_color` that would accept a `Color` or a `ColorCurve` in order to produce the env map image. That way, the `EnvironmentMapLight` could just have a simple `from_image` constructor that feels more idiomatic.