Various improvements related to height queries against an ellipsoid

[kring]

• Don't let EllipsoidTilesetLoader create tiles past level 30, because the integers used for our QuadtreeTileID will overflow at level 31.
• Tightened the tolerance of IntersectionTests::rayTriangleParametric, allowing it to find intersections with smaller triangles (such as the ones generated by EllipsoidTilesetLoader for a level 30 tile).
• Fixed a bug that could cause GltfUtilities::intersectRayGltfModel to crash when the model contains a primitive whose position accessor does not have min/max values (such as the ones created by EllipsoidTilesetLoader).
• Added new ITilesetHeightQuery interface. It may be optionally implemented by a TilesetContentLoader in order to provide a more efficient way to query heights. For example, EllipsoidTilesetLoader simply returns 0.0. Or, a tileset that has a corresponding web service to query heights could use that, instead of downloading and sampling tiles.

Fixes #994

[j9liu]

Thanks @kring ! Just tried this out in Unity and confirmed that sampling ellipsoid heights no longer causes a crash. Merging!