New ship/track collision system.

[unofficial-fish]

The old ship-track collision system has been thrown out. We now have these:

• ship_point_find_collision: Collision detection between points and tracks. Not bulletproof, (in junctions especially) but the original was not either, and it's easier to reason about.
• ship_resolve_collision: Collision response derived from the original code. Should behave identically.
• ship_resolve_collision_scrape: Basic "track scraping" collision response. Not written to match any particular entry, but could be elaborated upon.
• point_face_collision_t: Produced by collision detection and consumed by collision response.

Right now, ship_point_find_collision is passed a point and a section, checking the point against the section and all its neighbors, and selecting the shallowest collision. track_nearest_section might simplify that, but I haven't yet investigated whether that would be more robust or less.

[phoboslab]

You said “Not bulletproof” - is there any regression with this? Or is it "no worse" than what we currently have?

I don't like having functions that are not called just in case we might need them later. So ship_resolve_collision_scrape() should be used (with a menu setting to turn it off?) or not be in this PR.

[unofficial-fish]

I've fixed a bug: for some reason ship_collide_with_track was responsible for setting SHIP_DIRECTION_FORWARD, so rewriting it broke a few things.

You said “Not bulletproof” - is there any regression with this? Or is it "no worse" than what we currently have?

I had a carefully-trimmed-down 260 words here, but in the interest of brevity I'll say: this is somewhat subjective, and a rigorous comparison seems impossible. The data structures I have to work with are typical for the era and awful. I am trying to think of a flawless, performant solution, but it is not easy. More words available upon request.

I don't like having functions that are not called just in case we might need them later. So ship_resolve_collision_scrape() should be used (with a menu setting to turn it off?) or not be in this PR.

I've added a menu to switch between "authentic" and "experimental" collision resolution.