Added hydro flag `j`, restored bug with `i` for backw. compat.

[ohlidalp]

It turns out the i flag has a bug: it always makes the hydro invisible, but if it comes as first/single flag, then it also makes the hydro commanded by steering input. That means:

• ri will make the hydro respond only to airplane rudder, and invisible.
• ir will make the hydro respond to both airplane rudder and steering input, and invisible. This makes it behave a lot different than plain hydro though, and many SimMonsters mods use it as a feature.

To resolve backward compatibility, I added a new flag j, which makes the hydro invisible but without the quirks of i. The parser still recognizes i but internally I'm using only j and n.

The documentation https://docs.rigsofrods.org/vehicle-creation/fileformat-truck/#hydros needs to be updated:

• ` no flags given -> forcen` flag.
• n means Normal and makes the hydro driven by steering input.
• j makes the hydro invisible.
• i makes the hydro invisible, but BEWARE: if it comes as first/single flag, then it also forces n flag. This quirk is maintained for backwards compatibility.

Fixes #2932

[ohlidalp]

Test using this vehicle (all hydros have ir flags): dafsemi-hydro-ir.zip

• on master, it won't steer at all.
• on this PR, it will steer but differently than plain DAF.

Note it got broken in https://github.com/RigsOfRods/rigs-of-rods/commit/9421c6843c10973c835e2e44de730f6f5919405d - if you build an earlier commit, it will work the same way as in this PR.