Using the mapbox style file "mapbox://styles/mapbox/streets-v8" on mapLib and on itowns, I noticed the result as not at all comparable.

Thus I looked layers by layers to understand what were we doing wrong in itowns and tried to correct a maximum of bugs. The example using the mapbox stream has been added. (examples/vector_tile_2d_mapbox.html)

What this PR does:

• new feature:
  • support relative urls in style file (ArcGIS style file)
  • add a warn properties in source to add warnings and send them only ones
• itowns bug fix:
  • Mapbox stream
    • enter url in {z}/{y}/{x} instead of ${z}/${y}/${x}
    • wrong selection of metadata when "{id}" in sprite
    • gestion of MVT layer using the 'ref' properties
    • gestion of line and polygon Label (simplified -> we use the first point of the geometry and sending a warning)
    • good gestion of the layer.order properties
  • Style
    • take into account the zoom in style when displaying layer
    • no display of polygon when fill.color is undefined
    • no display of line when stroke.width is 0
    • no recoloring of icon when icon.color is white (to avoid a lightning of the icon)
    • allow multiple label with same text (for contour lines)
    • no display of icon if size is 0