This allows us to make materials that don't cast ray shadows.
Turning off this property can reduce the rendering time slightly.
Note: RNA path is changed to "use_cast_shadows" as well. The older
path "use_cast_buffer_shadows" still can be used as its alias, but
it will be removed after updating some addons.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D272