Make SharedAsset a base class, eliminate AssetContainer

[kring]

This is a PR into #959.

There's a corresponding cesium-unreal branch that goes with this:
https://github.com/CesiumGS/cesium-unreal/tree/shared-assets-image-cesium-only

@azrogers this is the refactoring we talked about (mentioned here). Once again it isn't completely done but is working well enough that you can build on it, if it seems like a reasonable direction to you. I renamed SingleAssetDepot to SharedAssetDepot, and AssetDepot to SharedAssetSystem. Feel free to merge into your branch when you're ready, or wait a bit on that if you prefer.

Some things I'm still concerned about (but don't know the best way to solve yet)...

Asset Factory

Currently, a Factory class is passed to getOrFetch. This is conceptually kind of broken. I mean, if I call that function twice with the same URI but a different factory, do I get the same resource both times?

Maybe it's a little bit academic, but if so, why not construct the SharedAssetDepot with the Factory instead, and save the trouble of passing it through each time? Well, we immediately run into the problem that we can't construct a factory for images without knowing the KTX options, which can technically be different on different tilesets (though it's not likely).

Intrusive reference counting feels error-prone

It's disastrous to create an ImageCesium instance on the stack or as a member of another class and then refer to it with an IntrusivePointer. If we do, after the last IntrusivePointer goes away, the instanced will be deleted, which will go very poorly.

This isn't really a new problem, but I think it became obvious to me while working on this. ImageCesium used to be a class that was usually instantiated on the stack or as a member of some larger object. But now it is reference counted and therefore must be heap allocated (i.e., allocated with new). There's a fair bit of possibility for error in the conversion.

I'm not sure yet how much to worry about this, or what we can do about it. One possibility is to prevent allocation anywhere other than the heap by making the constructor private and providing a factory function.

Random aside while I'm thinking about it: ImageCesium is a strange name. Perhaps ImageAsset would be more appropriate now.