Vala binding can't change map source

[supercamel]

Hi,

When using Vala bindings, I cannot change the map source. Further investigation shows that the osm_gps_map_set() function is never called (line 1977 of osm-gps-map-widget.c). This is because the old map source is -1 (less than OSM_GPS_MAP_SOURCE_NULL).

Changing lines 1642 and 2837 from -1 to 0 seems to fix the problem. I can't see any real reason why is the map source should be initialised to -1.

Cheers!