new equation in czm_alphaWeight shader function

[arneschilling]

The current alpha blending function used in Cesium in not suitable for our vegetation models.
In particular, we have problems displaying a large number of trees from I3DM if they are modelled as transparent billboards. Blended colors from overlapping textures are way too bright.

Changing the equation in function czm_alphaWeight fixed this problem.
The referenced paper in Shaders/Builtin/Functions/alphaWeight.js (http://jcgt.org/published/0002/02/09) mentions several alpha weight functions that can be used. Buildings and other geometries don't seem to be affected.
Equation (9) works best for us:
pow(a + 0.01, 4.0) + max(1e-2, min(3.0 * 1e3, 0.003 / (1e-5 + pow(abs(z) / 200.0, 4.0))))

comparison of alpha blending with current equation (8) - left and (9) - right:

[cesium-concierge]

Welcome to the Cesium community @arneschilling!

Can you please send in a Contributor License Agreement (CLA) so that we can review and merge this pull request?

⚠️ I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

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[pjcozzi]

Thanks @arneschilling!

There is a corporate CLA covering this. @arneschilling please add yourself to the virtualcitySYSTEMS GmbH section of CONTRIBUTORS.md.

[mramato]

Spoke with @bagnell offline, he's fine with this change (we both agree this is a big improvement, I've always need annoyed by it personally).

@arneschilling, please add an entry in CHANGES.md about this improvement and add yourself to CONTRIBUTORS.md (as @pjcozzi said above) and then we'll merge this.

Thanks!

[arneschilling]

okay, check

[mramato]

Thanks @arneschilling!