When cmgen creates ktx files, it creates a pair: one with an _ibl suffix
and one with _skybox. This convention is now honored by FilamentApp.
Note that we already have several KTX environments in the repo, so you
can clone a fresh repo and do:
gltf_viewer --ibl={FILAMENT}/docs/webgl/pillars_2k DamagedHelmet.gltf
Note that we have several hundred files in samples/envs, we should
consider replacing those or copying the environments in docs/webgl
(which git would internally de-dup, due to content hashing).
This change will allow the skybox use a compressed texture format.
For now, the old internal format "RGBM" is still honored, but in a
forthcoming change we will replace the enum with UNUSED.
