This is a re-attempt of 2c4b2a53 and
f9bc1037.

In contrast to the previous changes for `point_inside`, this does not make all
the static variables members of `inside_t`. The simpler ones, `vertex` and
`sides`, are just re-computed locally each time.

This is not a completely robust fix. It is possible there are callers who
(deliberately or inadvertently) zero the `inside_t` state every time they call
into these functions with the same nodes, inadvertently thwarting the functions’
state management. But there does not seem to be a better solution right now.

Gitlab: re-fixes #2498


Reported-by: Zhi An Ng <zhin@google.com>
Suggested-by: Stephen North <scnorth@gmail.com>