Environment maps: Add cubemap support alongside equirectangular images

In the explainer, it indicates the environment map is specified as an equirectangular image.

However GPUs are better suited at processing cube maps. They exhibit less artifacts around the poles and a UA may prefer or automatically convert an equirectangular image to a cubemap.

There are a few ways we could add cubemap support.

One is automatically, if the image is a file which supports layers and contains 6 layers, the image is automatically imported as a cubemap. Popular cubemap formats include ktx, exr, and dds amongst others.

Another option is specifying the cube faces directly as 6 different files. I.e., instead of:

environmentmap="envmap.hdr"

one could provide the maps for the 6 faces:

environmentmap-plus-x="envmap-x1.hdr"
environmentmap-minus-x="envmap-x2.hdr"
environmentmap-plus-y="envmap-y1.hdr"
environmentmap-minus-y="envmap-y2.hdr"
environmentmap-plus-z="envmap-z1.hdr"
environmentmap-minus-z="envmap-z2.hdr"

or alternatively we could support a comma separated list instead:

environmentmap="envmap-x1.hdr, envmap-x2.hdr, envmap-y1.hdr, envmap-y2.hdr, envmap-z1.hdr, envmap-z2.hdr"

These are just a few ideas, perhaps there are other better ones as well.

Start with the DOM API section of explainer.md and compare its current equirectangular environment-map definition with the cubemap approaches described here. The first step is to resolve which representation and file formats the API should support; done means one approach is agreed and the explainer specifies its syntax and behavior.