support for more environment map types

currently, blender supports "equirectangular" and "mirror ball" environment textures. while these are the most commonly used in film and vfx production, game engines are recently shifting towards octahedral environment representations.

Ocathedral environment maps have some advantages:

• they are square (-> more optimal in memory).
• they have a reasonably uniform texel density (-> better than equirectangular, similar to cubemaps).
• this makes them more suitable for p.e. creating roughness mipmaps for pbr shading.
• the conversion to and from a vector is not very expensive.
• on the downside, they are not as nice to look at when seen raw.

support for these as option in the "environment texture" shadernode, and the camera "panorama type" would allow export and import of these maps directly. Currently, these have to be converted in an additional step when the engine uses this type, introducting quality loss.

example octahedral map, converted from this equirectangular map: polyhaven.com/a/drakensberg_solitary_mountain example of the octahedral conversion code: shadertoy.com/view/MdsfzN