[Therion] Rock border not creating boulders with smooth altitude shading

[Tarquin Wilton-Jones]

> I'm with you Tarquin.
> See my other message (some reason it has not turned up in
> https://www.mail-archive.com/therion@speleo.sk/)
> Seems like it is intentional, but it is more complicated than that.
> This page describes
> https://therion.speleo.sk/wiki/contrib:externalviewers#examples_of_rendering
> _differences_between_pdf_viewers

I had not realised that this would cause me issues!

FWIW, I would prefer that rocks do not obscure a passage stacked below
their scrap completely. I do not mind seeing water as translucent below
a rock when the rock sits in the water.

I very much like the look of boulders as a translucent overlay on top of
water, since we have situations where boulders sit in water, and that
was a great way to represent them as a boulder with water underneath.
You could then use "placement bottom" to put a boulder underwater, such
as a shallow area within a lake. Either way, you could see a second
passage hidden underneath the first.

I do not care much for Adobe Reader and its inability to draw layered
opacity (which has caused issues for other applications that create PDFs
such as PrinceXML). It's an awful PDF reader.

Foxit, Sumatra, Evince; these look great to me, and the others the who
were discussing the output with me today.

> I had always thought double rendered translucent objects where the
> intention, but seems they are not.
> I would vote for making it an option (not withstanding what you actually see
> will depend on the viewer used).

+9
If someone uses Adobe, they get junk. So be it. Let good PDF viewers
display it nicely please. :)