Small optimization suggestion: skip updating errors on unmodified edges

[artem-ogre]

Thank you very much for this library! 🙇

In update_triangles it should be possible to skip re-calculating errors for the edges opposed to collapsed vertex:

Before

void update_triangles(int i0,Vertex &v,std::vector<int> &deleted,int &deleted_triangles)
{
    vec3f p;
    loopk(0,v.tcount)
    {
        Ref &r=refs[v.tstart+k];
        Triangle &t=triangles[r.tid];
        if(t.deleted)continue;
        if(deleted[k])
        {
            t.deleted=1;
            deleted_triangles++;
            continue;
        }
        t.v[r.tvertex]=i0;
        t.dirty=1;
        t.err[0]=calculate_error(t.v[0],t.v[1],p);
        t.err[1]=calculate_error(t.v[1],t.v[2],p);
        t.err[2]=calculate_error(t.v[2],t.v[0],p);
        t.err[3]=min(t.err[0],min(t.err[1],t.err[2]));
        refs.push_back(r);
    }
}

After

void update_triangles(int i0,Vertex &v,std::vector<int> &deleted,int &deleted_triangles)
{
    vec3f p;
    loopk(0,v.tcount)
    {
        Ref &r=refs[v.tstart+k];
        Triangle &t=triangles[r.tid];
        if(t.deleted)continue;
        if(deleted[k])
        {
            t.deleted=1;
            deleted_triangles++;
            continue;
        }
        const int vi0=t.v[r.tvertex], vi1=t.v[(r.tvertex+1)%3], vi2=t.v[(r.tvertex+2)%3];
        t.v[vi0]=i0;
        t.dirty=1;
        t.err[vi0]=calculate_error(i0,t.v[vi1],p);
        t.err[vi2]=calculate_error(t.v[vi2],i0,p);
        t.err[3]=min(t.err[0],min(t.err[1],t.err[2]));
        refs.push_back(r);
    }
}

Please let me know what you think.