Shadow aliasing in Chrome

[lilleyse]

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

[mramato]

@lilleyse can you submit an actual chrome bug to https://bugs.chromium.org/p/chromium/issues/list and mark it as an ANGLE issue there (it should be one of the choices). If it's easy to have a reduced test case, definitely include one. After that we can reach out to some of the ANGLE/Chrome team and ask them to have a look.

[lilleyse]

Updated bug report here: https://bugs.chromium.org/p/angleproject/issues/detail?id=1608

The only time this bug surfaces in Cesium is when running ANGLE and not supporting the depth texture extension. So the good news is that percentage of users is pretty small, but it includes me...

I'll update Cesium as well since currently polygon offset is turned off for all ANGLE users but it doesn't need to be.