Function game::parse_center_camera

fn parse_center_camera(
    ctx: &EventCtx<'_>, 
    map: &Map, 
    raw: String
) -> Option<(Pt2D, f64)>

Parse an OSM-style zoom/lat/lon string (https://wiki.openstreetmap.org/wiki/Browsing#Other_URL_tricks), returning the map point to center on and the camera zoom.