Round latitude and longitude values in links to a sensible number of

Round latitude and longitude values in links to a sensible number of
decimal places for the zoom level whilst still preserving accuracy
sufficient to position the map within a few pixels.

The relationship between zoom level, decimal places, and the accuracy
with which the map can then be positioned in pixels is as follows:

  Zoom  Decimals  Accuracy (pixels)

   1       0            1.42
   2       0            2.84
   3       1            0.57
   4       1            1.14
   5       1            2.28
   6       2            0.46
   7       2            0.91
   8       2            1.82
   9       3            0.36
   10      3            0.73
   11      3            1.46
   12      4            0.29
   13      4            0.58
   14      4            1.17
   15      5            0.23
   16      5            0.47
   17      5            0.93
   18      6            0.19

So the worse case is at zoom 2 where accuracy is still within 3 pixels.

Based on a patch by rjmunro.