feat(map): normalize the dpi to 96

[mbarbeau]

What is the current behavior? (You can also link to an open issue here)

The dpi of the layers depends on the data source:

• WMS 1.3 standard specifies the pixel width : 0.28 mm. That value gives a resolution of 25.4/0.28 = 90.71 dpi approximately.
• Only GeoServer respect this standard
• MapServer by default has a dpi of 72 but can be modified in the mapfiles
• ArcGIS Server / Qgis Server by default has a dpi of 96

Some visibility issues are present due to poor resolution.

What is the new behavior?

Popular WMS servers extend WMS service with dpi parameters:

• MapServer with MAP_RESOLUTION=value
• GeoServer with FORMAT_OPTIONS=dpi:value
• ArcGIS Server / QGIS Server with DPI=value

Common GIS softwares works with a resolution of 96 dpi. For example, QGIS adds all those parameters at the end of WMS/GetMap queries.

IGO is now doing the same way.