Environment maps: Add cubemap support alongside equirectangular images

[mwyrzykowski]

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.