Send the DPI value to GetMap requests

[jahow]

Geoserver supports a format_options=dpi:... parameter in WMS getmap requests: https://docs.geoserver.org/latest/en/user/services/wms/vendor.html

Mapserver supports map_resolution=..., see https://mapserver.org/cgi/controls.html#using-mapserver-version-5

QGIS supports DPI=..., see https://docs.qgis.org/2.18/en/docs/user_manual/working_with_ogc/server/services.html#id2

I guess inkmap could send all three formats in the request, hopefully that won't cause any errors (doubt it)

This is really valuable since this would mean having high DPI prints with readable text/symbols when using WMS