Fully support specifying a custom ellipsoid

[azrogers]

cesium-native already allows specifying a custom ellipsoid with most of its operations. This change removes the few places where the WGS84 ellipsoid was hardcoded, allowing all of cesium-native to be used with other ellipsoids. This is necessary to support CesiumGS/cesium-unreal#366.

[kring]

Looks great @azrogers, just a few comments below.

[csciguy8]

Thanks for the changes @azrogers !

From what I can tell, all review comments have been resolved. I'll give @kring another chance to chime in though...

One clarifying question from me, do we still need CESIUM_DISABLE_DEFAULT_ELLIPSOID? If not, can we remove it? And if so, is it useful enough to put in our release notes so that other developers can use it?

(Btw, I appreciate the sheer number of files you needed to change for this PR, quite impressive!)

[kring]

One clarifying question from me, do we still need CESIUM_DISABLE_DEFAULT_ELLIPSOID? If not, can we remove it? And if so, is it useful enough to put in our release notes so that other developers can use it?

I asked Ashley to add this because I think it's very useful, such as when adding alternate ellipsoid support to a new product that uses cesium-native. Adding it to the release notes seems like a good idea.

[kring]

Getting close now!

[azrogers]

@kring Updated the PR based on the review. BoundingRegion, S2CellBoundingVolume and Tileset no longer store copies of the ellipsoid, which required a good number of changes.

[kring]

A few more small changes, I think this should be the last of it!

[kring]

Thanks @azrogers!