Made GlobeSurfaceTile.pick not pick triangles behind the ray

[IanLilleyT]

Introduced this bug in #8390 (mid-December 2019). The old way uses IntersectionTests.rayTriangle which returns undefined when the triangle is behind the ray. The new way uses IntersectionTests.rayTriangleParametric which returns a negative distance when the triangle is behind the ray. Since I never added a check for this in GlobeSurfaceTile.pick, triangles behind the ray could be picked. I added a comment and another picking test to avoid this confusion in the future.

Fortunately this bug never made it into an official release.

@lilleyse Let's get this in before the January release.

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

• ✔️ Signed CLA found.
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Reviewers, don't forget to make sure that:

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

[lilleyse]

Looks good, thanks @IanLilleyT