Allow user to convert single lat-long hdri into cubemap

[jkinz3]

HDR images (like those downloaded from polyhaven.com) come in lat-long formats that need to be converted into cubemaps. Tools to do so that are frequently cited are often outdated and not supported.
Basically, I think DirectXTex should offer similar functionality as the gltf ibl sampler
I would argue that DirectXTex should offer the ability to generate the PBR ibl textures from the resulting cubemap, but that can be considered more debatable. At the very least, it should be able to create a cubemap from a single lat-long file.

[walbourn]

I have been considering a texenv tool for some time to do various kinds of cubemap operations. I'll keep this in mind if I find time to get back to it sooner than later :)

[walbourn]

This seems like functionality that should be in texenvmap.

#509