Better handle map boundary conditions

[abey79]

We currently handle very poorly boundary conditions (e.g. what happens between longitude +179° and -179°). This should be improved, and would involve at least the following:

• walkers: have the zoom=0 tile infinitely repeating and scrollable horizontally (like OpenStreet Map).
• When sufficiently zoomed out, repeat draw all objects as many times as required.
• Define GeoLineStrip as "connecting the dots" using the shortest distance in the EPSG:4326 space (e.g. d = hypot(lat_diff, lon_diff)) and properly handle the wrapping accordingly.
• Properly handle wrapping in GeoSpan.