make Arc a vertex-source, allow it to be composed into paths..

I typed arc as an IVertexSource (since it already conforms), and added a flag moveToStart which controls whether it moves or draws to the arc start-point, This allows it to be composed as an arc into another path. For example, a pie-wedge which wants to include the origin point, like this...

var path = new PathStorage();
path.MoveTo(originX,originY);
path.concat_path(  new arc(
     originX,originY,
     radiusX,radiusY,
     DegreesToRadians(startAngleDeg),
     DegreesToRadians(endAngleDeg),
     moveToStart:false));
path.LineTo(originX,originY);