Shadow aliasing in Chrome

The polygon offset setting for `ShadowMap` has very little effect in Chrome. As a fallback, the shadow cast shader pushes depth values linearly with camera distance, which is prone to light leaking.

I first noticed this with the transition from Chrome 49 to Chrome 50. Polygon offset works fine when ANGLE is disabled.

More details here: https://groups.google.com/forum/#!topic/webgl-dev-list/E1dAG65QBhg

Start with the ShadowMap polygon-offset behavior and the shadow cast shader, then reproduce the difference between Chrome 49 and Chrome 50 with ANGLE enabled and disabled. Read the linked WebGL discussion for context. Done means polygon offset has a meaningful effect in Chrome without relying on the camera-distance fallback that can cause light leaking.

Written by the indexing model from the issue text.