Fix incorrect distanceSquaredTo calculation in BoundingSphere

[j9liu]

As noted in the forum, the distanceSquaredTo calculation of BoundingSphere was incorrect. It was calculated as (position - center)**2 - radius**2, instead of ((position - center) - radius)**2. This PR fixes the algorithm and the unit test that corresponds to it.

cc @lilleyse @ebogo1

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

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

[lilleyse]

Thanks @j9liu for the fix, and thanks @donhatch for letting us know about the problem in the forum post and in #9534

Looks like this problem goes way back: febb47b

Turns out this function is only used for sorting translucent commands and sorting tiles for globe picking which have managed to get away with less than perfect results. Thankfully SSE calculations are correct since they use TileBoundingSphere.prototype.distanceToCamera which is actually implemented correctly.

Just a couple suggestions

• It should return 0.0 if the point is inside the sphere. OrientedBoundingBox does the same thing. This should be mentioned in both classes' documentation like it is in TileBoundingSphere.prototype.distanceToCamera. Add a test for this.
• In the BoundingSphere doc remove the word "estimated". It can just be "The distance squared from the bounding sphere to the point"
• Looks like the OrientedBoundingBox doc copied the same line "The estimated distance squared from the bounding sphere to the point." It should be "The distance squared from the oriented bounding box to the point."

[j9liu]

@lilleyse - Thanks for the review. I just made some changes that addressed your points, let me know if I missed anything!

[lilleyse]

Thanks! Looks good.