该对象是否投射阴影？
ShadowCastingMode enum defines how and if shadows are cast from this object.
Typically shadows are either cast or not, but it's also possible to make shadows two-sided
(useful for otherwise single-sided geometry) or make a shadows-only object (that is
otherwise invisible in the Scene, but casts a shadow).
另请参阅：ShadowCastingMode、receiveShadows、Light.shadows。