Re-triangulate the spot shadow.

Fix the valid umbra detection.

This looks better b/c every vertex will have one ray shooting at it, such that
we don't miss the corner.

This performs better too, due to the polygon intersection is removed and less ray
intersection. 2x performance for rect and circle for spot shadow in test app.

    b/17288227
    b/15598793
    b/16712006

Change-Id: I4a5ee397b9e192e93c8e35e6260b499e3e38a6f4