Allow overriding pixel size

[IvanSanchez]

Fetching the pixel size from the CSS attributes is a guess at best, and has no way of working in some scenarios (projector screens with unknown distance to the wall).

It should be sensible to add an option for overriding the size of a pixel.

[lvisei]

how to input scale(1:1000/etc) get map zoom number in leaflet ？

@IvanSanchez @MarcChasse