sqeeze in a minor bugfix.
I wouln't be surprised it is too late,
but since it really is sort of minor, it doesn't matter really.
I had mistakenly put the export of dispersion and absorption parameters in the
ray_mirror block instead of ray_transp, which means that both dispersion and
absorption can only be used if both ray mirror and ray transp are enabled.
Anyway, I don't want to make everyone recompile release builds if already
done, it is minor, but users should take this into account at least.
e4e096187e