2D Occluders with cull mode enabled show light before the shadow

Ubuntu 17.04 - Godot 3 alpha

Issue description: Enabling the cull mode on LightOccluder2D shows some light where the shadow should start

The right one is counter clockwise and light penetrates a bit the occluder

Steps to reproduce: Enable a culling mode on a LightOccluder2D. Can be tested with "light and shadows" demo, using culling mode in clockwise.