Flattened Cubemap view

[JasperCarmack]

Describe Your Suggestion

The ability to see all 6 of the cubemap faces in a preview would be nice, especially for debugging cubemap renders/searching for issues with rendered cubemaps. That way a user doesn't have to manually set the face number to see each one. A similar approach could be used for skyboxes to see how they look unwrapped.

[NvC-DmN-CH]

Kinda related, but it would be cool for cubemaps/skyboxes to be easily editable with tools like these but local:

https://jaxry.github.io/panorama-to-cubemap/

https://danilw.github.io/GLSL-howto/cubemap_to_panorama_js/cubemap_to_panorama.html

[craftablescience]

Going to make another issue with the relevant skybox stuff

Edit: #150