Selectively disable "cast shadows" on scene objects

[taiya]

Reproduction steps – use large sphere world (like for examples/shapenet.py) – use directional lights (like for helloworld) – rendering is just black images...

Tentative fix: add option to "disable cast shadows" (exists in blender)?

[taiya]

@Qwlouse as discussed in #159

[boomboomluo]

Hello, I want to completely eliminate the ground "cast shadows" in MOVi dataset. Counld you please tell me how to make it?