Support CORS in Cesium's development server

At least when it is running locally.

After

app.use(compression());

add

app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
  next();
});

[bampakoa]

@pjcozzi I would like to take on this, if is still relevant.

[bampakoa]

@hpinkos You were right! I wasn’t testing it properly. Thanks for the info.