Add support for outlines on path layers
Submitted by Marcus Lundblad
In the new Maps code for rendering transit itineraries we draw route lines color from the transit timetable data, when available. To avoid getting hard-to-see situations with very light-colored lines against the map background we draw a an outline around the path for lighter colors. Currently this is done in a kinda hackish way by drawing two paths on top of each other with different widths (the wider one first using the outline color and then the other color is drawn on top) as can be seen here: https://git.gnome.org/browse/gnome-maps/tree/src/mapView.js#n615
It would be nice to have a way to do this directly in libchamplain. Something like having two extra properties "outline-width" and "outline-color" to set the width of the outline border and color respectively.