[Therion] thfill with page background colour?

[Tarquin Wilton-Jones]

> In which variable do i access the page's background color (the one set
> wit the layout option "map-bg")?

Fun fact ... you can't access that. Or at least, it didn't seem to be
available back when I was trying to do that in version 5. And I still
can't see any useful variable in version 6.

"background" is the default background (white) in v5, but even that is
absent in v6.

I tested like this with identical input using the -d switch to get the
data.mp:

1. color map-bg [100 100 0] # yellow
2. color map-bg [100 0 0] # red

The data.mp debugging output was identical in both cases, so it is not
putting the colour into a variable in that file.

In places where I have needed this, my layout included a "code metapost"
which set a global variable where I specified the background colour
(written in metapost notation this time). The symbol code could then
read my global variable. Messy solution.