Equirectangular HDRI skybox

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

None of the HDRIs or panoramas out there seem to be available as cube maps.
The tools for conversions mostly don't work or have errors when building.
Even with having cube maps it won't solve the issue of loading an equirectangular HDRI at runtime.

## What solution would you like?

It would be great for bevy to either have it's own way to render an equirectangular pano as a skybox
or convert the equirectangular map to cube map and then render the cube map.

## What alternative(s) have you considered?

All of them, none solve rendering equirectangular panorama at runtime.

## Additional context