SampleDistantLight: normal setting makes wrong shadow

[cedarz]

In DirectLight, as the comment says, lightSample.normal is required for quad light.

if (dot(lightSample.direction, lightSample.normal) < 0.0) // Required for quad lights with single sided emission

for distant light, if the position is not high(in y axis) enough, there will be wrong shadow cast on the scene plane, such as teapot.scene.

Changing the lightSample.normal compution from

lightSample.normal = normalize(scatterPos - light.position);

to

lightSample.normal = -lightSample.direction;

maybe one solution that bypasses the problem.