Filament will now allow incomplete mipmap chains for the IBL and
map the roughness to the available chain.
This allows to create roughness cubemaps with a minim size for
roughness==1, instead of always mapping it to 1x1.
cmgen will now use 16x16 cubemaps for roughness==1, which
improves the quality of rough objects significantly.
The beauty of this is that there is no asset or API change. Old assets
will continue to work like before.
dbda77b091