the skipvoid function may miss the true entry position

[ShijieYan]

Based on the current implementation, a photon is moved '1' unit length forward before it lands inside a non-zero voxel and get traced back to the entry point of that voxel. The true entry position could be missed, as shown in the figure,

This is even more problematic in SVMC mode because it allows a photon propagate inside 0-labeled regions if a split voxel.