- Make all members constant, and pass an anonymous temporary fvi_query
to find_vector_intersection.
- Change `p0`/`p1` to `const vms_vector &`, since the positions are
mandatory. Callers can no longer pass `nullptr` or an uninitialized
value here.
- Change `thisobjnum` to `icobjptridx_t`. Calls to fvi_sub built an
objptridx at need, so moving it to the caller allows it to be
constructed once per find_vector_intersection call.
- Move `flags` and `rad` out of fvi_query, since calls to fvi_sub may
use other values than the ones in fvi_query. This prepares for
passing fvi_query to fvi_sub.
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