Fix some uses of part_at() to use precalc positions

[Kamayana]

part_at() refers to parts by precalc, unlike most other vehicle functions that go by mount, so part_at was always returning -1 in some places

Summary

None

Purpose of change

While most vehicle functions that fetch a part need the mount point as a parameter, part_at() uniquely needs to be given the part's precalc position. This made some uses of part_at always return -1 in some places, such as for appliances (which can have inaccurate precalcs because they never move and thus, never update them).

Describe the solution

Add a coord_translate to convert the mount point to a facing-relative direction, which is equivalent to precalc[0].

Describe alternatives you've considered

Rewrite part_at to use mount points instead. It gets used in tandem with get_points() though, which also uses facing-relative positions, and that's a much wider-used function.

Testing

Added a debug message, checked the part index found:
Before:

After:

Additional context