It is now. In fact, as of the current version (as of this writing, v. 3.5.1), you can't use a cubemap or a six-sided image since this is no longer a Background option in WorldEnvironment: it must be an equirectangular panorama image, so you have to use a converter. Experimentation has shown me that a panorama image sized 1280 x 720 works well for 3D environments. Each of your six source faces should be 1024 x 1024 (if you're starting from a cubemap).