Grid box selection not working between 1W and 0 degrees longitude

[ebarry]

Continued from https://github.com/publiclab/plots2/issues/6697#issuecomment-555565550

The Problem

In the map picker, i was unable to choose the square for London, see gif below. The problem with the red highlight "skipping" over certain areas seems to be happening to certain longitudes, namely between 1W and 0 degrees longitude. Is there perhaps a math error involved? @jywarren observed that:

the algorithm to draw the boxes is probably is not set up for the special case of a box where one longitude is ~359 and the other ~0. Thanks!