Pixel ratio validation fails

When dealing with imagery from far up north(like Norway) pixel ratio can well exceed boundaries defined in [this code](https://github.com/mapbox/mapnik-omnivore/blob/master/lib/raster.js#L54).

Is there any background to why chosen max value is 2? For the imagery that I'm having problem with(can provide example if needed) ratio is over 3, but it seems to be a perfectly valid geotiff.

cc @GretaCB

Start in lib/raster.js at the linked pixel-ratio validation around line 54, then inspect how GeoTIFF metadata is read and checked. Confirm the reported ratio for imagery from far north and determine whether the fixed maximum of 2 is intentional. Done means the validation correctly handles valid ratios above 3, with coverage for the reported case if the repository has relevant tests.

Written by the indexing model from the issue text.