Store the coordiantes(lon/lat) of the center of the scaled primary buffer, and use it in `drawPrimaryScreen`

[ClaraSchumann]

Now it works with greatly enhanced robustness, panning around immediately after a zoom-in/ zoom-out will not result in striange unalignment.

[ClaraSchumann]

I will continue to contribute these days. I will work on the network proxy, and try to refine the loading scheme. Now it is too aimless.