This adds createInstancedAsset() to AssetLoader, which creates a master
asset and a set of slave instances. Vertex buffers, index buffers,
textures, and material instances are shared. Entities and components are
duplicated.
Instances have their own API object that is very simple. Light sources
and material instances are not instanced, so are not accessible through
the instance API object.
The master-slave ownership model lets us avoid complex shared ownership
semantics. The existing cache structures in AssetLoader allow the
implementation of this feature to be fairly simple.
The master asset exposes the union of all entities and allows clients to
modify all instances en masse if they wish. This design also works
naturally with ResourceLoader, which does not need to know about
instancing. For example, asynchronous loading is completely unchanged;
the dependency graph simply contains the union of entities across all
instances.
Support for animation is added in a subsequent commit.
Fixes#1513.
8de730f76c