Consider improving raytrace return values

[sprunk]

Trace rays (GuiTraceRay, various CGround methods etc) return a float, which is the distance to ray intersection point.

• the ray can fail to hit anything, in which case the distance is -1.0f. The type doesn't make this magic value obvious and it can ruin math if you forget to take it into account.
• if the ray hits something you have to recalculate the intersection point from scratch via distance * normDir + origin.
• hit unit/feature is "returned" by the caller passing a reference, which is confusing and has to be passed even if you're not interested in the values. The types involved also don't make it obvious that they are exclusive since you always pass both.

In that vein how about making it more obvious? Something like (named definitions just for exposition):

using GroundOrWater = bool; // or maybe even std::monostate for a general "some surface"
using WhatWasHit = std::variant <CUnit*, CFeature*, GroundOrWater>;
using Distance = float;
using HitLocation = float3;
using Hit = std::tuple <Distance, HitLocation, WhatWasHit>;
using TraceRayResult = std::optional <Hit>; // empty if didnt hit anything