Map view: ensure data extent is set on start

[JakobMiksch]

• before when the map was loaded the extent was initially at null island and jumped afterwards to the extent of the data. see GIFs below. I find this a bit distracting and confusing.
• this PR ensures the map has the data extent directly at the start by loading the data first and initializing the map afterwards
• for loading the data the very ancient XMLHttpRequest was chosen to guarantee maximum browser compatibility. However we could also use the more modern fetch approach.

before:

after:

[dr-jts]

What if the data takes a long time to load, or errors out?

[JakobMiksch]

• loading the in-built example with 10000 points loads in reasonable time (a few seconds):

• when I add a 5 second timeout in the JavaScript code, the map stays white until there is data to display, see here:

• I have not simulated a complete timeout ( so that there will be no data returned at all), but I guess then the map will stay white forever ...

What would be your preferred behavior in this case?

[JakobMiksch]

@dr-jts I pulled the latest commits from master and it still works as expected.