feat: new_map centers automatically control point on negative coords

[AzazelN28]

Un pequeño programilla para probar que funciona la feature:

program feat_new_map_center;
private
    int cx, cy;
begin
    graph = new_map(32, 32, -1, -1, 0);
    get_point(file, graph, 0, offset cx, offset cy);
    assert(cx == 16, "Punto de control inicial en el centro");
    assert(cy == 16, "Punto de control inicial en el centro");
    exit("Ok", 0);
end

function assert(condition, message)
begin
  if (!condition)
    exit(message, 1);
  end
  return(0);
end