bucket: Make no bucket cross the 0 and 180 deg longitude border.

bucket: Make no bucket cross the 0 and 180 deg longitude border.

Change the bucket tiling at the poles not to cross the
0deg longitude and not to cross the 180deg longitude line.
This lets some special cases disappear for the buckets as well
as for the hierarchical level of detail spt loader.
Also change the last degree tiles from spanning 360 degrees to
12 degrees. Before we had 8 nested rings which neither helps
scenery paging nor culling. With that change the chunks are
about the same order of size than any other tile. Also the
tile shapes are much more friendly for culling and paging.
Also in presence of the current scenery, this polar tiles contain
as of today just totally broken geometry which tells me that
also the scenery generation process did not like these rings
for the tile shapes.

The impact on the current scenery is low. With this change the
polar regions do no longer load the tiles that are available
in the current scenery builds. The last degree tiles are broken
in this scenery version anyway. The next degree that is changed
will really loose some tiles that were sensible as of today.
All these areas are replaced with sea tiles by the old paging
structure. So, you will have at least ground below you when you
fly there. A hopefully comming soon scenery rebuild will
use this new tiling structure and thus will be the first one
providing closed polar caps.