Propagate DirectSample{Light,Sensor} setting to PathVertexContext

Feature-Start: bdpt-mis-direct-lighting
Use separate subpaths when direct sampling is used in BidirectionalPathTracer

This is necessary since some methods work on the entire subpath instead
of just the last few vertices (for example,
PathVertex.ComputeExpectedWeight()).
Use the right pdf values when direct lighting is used in PathVertex
Re-enable direct lighting for all BDPT scenes

Feature-End: bdpt-mis-direct-lighting