Take light shape into consideration and do a small
fade to avoid abrupt lighting changes.
This fixes quite a lot of light leak but doesn't
fix all of them since light can still leak
during the fade. We could do this fade on the
lit side but then it break the working cases.
This does however fix the appearant sharp shadowing
that was visible on big light source.
Co-authored-by: Weizhen Huang <weizhen@blender.org>
763b3b3b12