The web build now invokes cmgen three times for each IBL: uncompressed,
S3TC (desktop), and ETC (mobile). Alternatively, we could enhance cmgen
to accept multiple compression strings, but per discussion with Mathias
we decided to keep it simple.
I decided not to use compression for the skybox in our web demos because
the quality is too poor (see PR image). I did not try playing with
compression settings, might be worth revisiting in the future.