[Therion] Hatching profiles and extended elevations

[Tarquin Wilton-Jones]

> for the hatches:
> maybe you could do a custom area symbol and use it with "clip off"
> option. I don't know if this is supported, but it might work. You could
> try it with some other area first...


Another option is to make the wall:bedrock line have rays pointing at an
angle (using a strict angle, not the angle of the line) but cutafter the
line itself. But I think this might be extremely tricky to implement,
and I can't think of an an easy way right now. The issue would be tight
corners, where it is simply not possible to do all the clipping correctly.

You could though define an area between the wall line itself, and a path
that you calculate (using sharp corner avoidance) outside the wall line:
https://therion.speleo.sk/wiki/metapost#shuttering_two_parallel_lines
Then fill that area using a hatching pattern. That could actually work,
and not be totally insane. However, your issue would be when you change
from one line to another - at that point, the fill areas might not match
up perfectly, and you might get unhatched gaps. It would only work for
lines that you have redefined to have that cool fill pattern. Maybe
someone will feel inspired to make a proof of concept...