Query Seamless Cubemap Support for WebGL1

FireFox Nightly recently introduced seamless cubemaps, and enabled it in a way that works in WebGL1 contexts. It seems like other browsers (Chrome, Edge) intend to match this behaviour, which means this feature might be usable soon in a lot of WebGL1 apps.

This is good news for physically based rendering, since ThreeJS is currently having to work around the cubemap seams by jumping through various hoops (packing LODs and faces into a 2D texture and manually interpolating).

It would be neat if there was a function or reliable way to query/detect for this feature in the WebGL1 contexts, so that ThreeJS and other engines could use an optimized path when possible, e.g. just a single texture fetch with textureCubeLodEXT.