change default precision to 1/1000th of viewbox diagonal

[anthrotype]

this improves the warping of very long segments that span across the wave such as the flags' rectangles.
It doesn't increase the file size overall, since our initial parametrization is good enough that we very rarely end up exceeding the precision threshold

E.g. see this detail from the serbian flag, notice that the crown sits exactly at the limit between the red and blue horizontal stripes:

After warping with the old default precision (1/200th of diagonal), the position of the emblem looks slightly off (notice the blue strip)

With --precision=1000 it looks better and the emblem is perfectly aligned: