Full support for multiple-b3dm-in-cmpt tiles, and some other fixes

[kring]

Builds on #102, #103, and #106, so merge those first.

• Add full support for cmpt files that contain multiple b3dms. This works by merging the multiple glTFs together, a reasonably straightforward operation. I considered an alternate designs where a tile can contain multiple glTFs, but this forced more multiplicity (and thus more code) on clients (e.g. cesium-unreal) for no discernible benefit, because glTF already has all the multiplicity we need. I also considered allowing a single 3D Tiles "content" payload to produce multiple sub-tiles, but that got awkward with tiles that also have actual child tiles. Merging glTFs was clean, easy, and performant.
• Fixed a bug that could cause runaway refinement when zooming close to a tileset with raster overlays (i.e. Cesium World Terrain). This also reduces the number of unnecessarily-upsampled tiles, making cesium-native on par with CesiumJS.
• Improved the approach for calculting the bounds of upsampled tiles so that it doesn't compound floating-point error. The compounded error could cause too many raster overlays to be mapped to geometry tiles, which is not great in general but is especially bad for cesium-unreal with its low max number of raster overlays.
• Added the tile URL to the extras of loaded glTFs. cesium-unreal uses this to name the ActorComponents created from the glTFs, so that when you click on a tile in the editor, the URL of the tile is printed in the console.
• Update: this now also includes the ability to upgrade the data type of indices decoded from Draco when necessary, fixing a rendering problem in the San Miguel tileset. Thanks to @baothientran. (see Upgrade draco index #110)

Fixes #104

[kring]

This is currently a PR into the gltf branch to make it easier to see what changed. It should be retargeted to master once #106 is merged.

[baothientran]

@kring this looks good to me. I only has a small comment below. After that, I think I can merge it into gltf branch

[baothientran]

it seems to be a typo. Is it supposed to be 16?

[kring]

Good catch! Fixed.

[baothientran]

thanks @kring! San-Miguel model works correctly