[Therion] New symbols

[Tarquin Wilton-Jones]

> I have some comments on your symbols. First of all - Very nice! I see
> considerable time expended, and it shows in the results.

Totally agree, some really good stuff in there. That cracked mud looks
awesome :)

> A few of these symbols would be useful to me. The symbols for danger,
> cracked mud (both point and area) and masonry have immediate application
> in some maps I am working on.

If it helps, I do already have a p_u_warning on the Metapost section of
the wiki.
https://therion.speleo.sk/wiki/metapost#warning_triangle
Same idea, different style. (And I have a newer version to update it
with...)

> What is "ex-voto"? The symbol you created looks to me like a pair of
> syringes. That is not something commonly found in a cave!

A shrine. I think they are supposed to be crutches - if you are
disabled, you might leave an offering at the shrine?