Improve the spot shadow computation.

Get rid of compuation of the intersection for penumbra and convex hull for umbra.
Use simple circle / normal to compute the penumbra and simple intersection for umbra.

The new way could be 2x to 4x faster from rectangle to round shape.
And this part is roughly half of the shadow computation, or 2/3 of spot shadow
computation.

This improve the spot shadow spikeness too.

b/16712006
b/14976551

Change-Id: I02911784868731369efa73f76fc915bc08248600