// Same half-cell samples and last-free result as the engine's CPU raytrace.
// ngp_solid_at supplies terrain only; rigid bodies are outside this API.
bool ngp_solid_at(vec2 position);
vec3 ngp_surface(vec2 start, vec2 end) {
    float distance=length(end-start);
    if(isnan(distance)||isinf(distance)||distance>512.0) return vec3(start, -1.0);
    int steps=max(1,int(ceil(distance*2.0)));
    vec2 previous=start;
    for(int k=1;k<=steps;++k) {
        vec2 point=start+(end-start)*float(k)/float(steps);
        if(ngp_solid_at(point))return vec3(previous,1.0);
        previous=point;
    }
    return vec3(end,0.0);
}