Incorrect zoom for some bounds

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For some bounds it calculates incorrect zoom, for example here `[-112.16601, 37.622623, -112.082016, 37.672085]` it calculates zoom 13 and it doesn't include most of markers, correct zoom will be 12.

`geoViewport.viewport([-112.16601, 37.622623, -112.082016, 37.672085], [672, 376])`

Here is example URL for static map with zoom 13:
https://api.mapbox.com/styles/v1/tripexpertweb/ckcdpjifn06ru1impw321xsqi/static/pin-s+000(-112.15432,37.672085),pin-s+000(-112.082016,37.624809),pin-s+000(-112.16601,37.622623),pin-s+000(-112.15432,37.672085),pin-s+000(-112.15432,37.672085),pin-s+000(-112.15432,37.672085),pin-s+000(-112.15611,37.64101),pin-s+000(-112.15611,37.64101)/-112.12401330471039,37.64735841514176,13/672x376@2x?access_token=pk.eyJ1IjoidHJpcGV4cGVydHdlYiIsImEiOiJja2Fud3l2MDcwbXZ0MndyeGF3eGI3ZWhzIn0.zLqNa1RECyZZadF8zfQM5w

And correct one with zoom 12:
https://api.mapbox.com/styles/v1/tripexpertweb/ckcdpjifn06ru1impw321xsqi/static/pin-s+000(-112.15432,37.672085),pin-s+000(-112.082016,37.624809),pin-s+000(-112.16601,37.622623),pin-s+000(-112.15432,37.672085),pin-s+000(-112.15432,37.672085),pin-s+000(-112.15432,37.672085),pin-s+000(-112.15611,37.64101),pin-s+000(-112.15611,37.64101)/-112.12401330471039,37.64735841514176,12/672x376@2x?access_token=pk.eyJ1IjoidHJpcGV4cGVydHdlYiIsImEiOiJja2Fud3l2MDcwbXZ0MndyeGF3eGI3ZWhzIn0.zLqNa1RECyZZadF8zfQM5w

If I convert back to bounds from center and zoom, it doesn't match initial bounds:
`geoViewport.bounds([-112.12401330471039, 37.64735841514176], 13, [672, 376])`

`[-112.1817398071289, 37.621845878167704, -112.0663833618164, 37.67295135774715]`

In this case `[2.269904, 48.849976, 2.391055, 48.874565]` it calculates zoom 12 which is correct and includes all markers.

Start by reproducing the reported cases with geoViewport.viewport and geoViewport.bounds, using the supplied bounds and viewport size. Trace how the library converts bounds to center and zoom, then verify that the Utah case returns zoom 12 and includes the markers while the Paris case remains correct. Done means the regression is covered by a test and both conversions agree with the expected bounds.

Written by the indexing model from the issue text.