Switch to two-pass shadow volumes for classification

[lilleyse]

This changes classification from the 3-pass stencil preload method to the 2-pass zfail method described here.

#8854 needs to be merged first because it clamps the shadow volume to both the near and far planes which is a requirement for the 2-pass method. The 3-pass method only requires that geometry is clamped to the far plane.

In most cases this reduces shadow volume fillrate by 33% which should translate to better performance in scenes with many ground primitives.

What I tested so far:

• Overlapping primitives
• Concave primitives
• Textured ground primitives
• Ground polylines
• Classification on 3D Tiles
• Invert classification
• Vector tiles
• Multifrustum

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

I'm seeing an improvement from 32 fps to 42 fps on the vector tile classification sandcastle: link

[IanLilleyT]

Looks good! I tested everything myself with multi-frustum and orthographic and couldn't find any problems. The performance boost is great!