renderer: batch object spawning

[marceline-cramer]

Objects lazily load their assets when they're spawned, so when you spawn in several objects in sequence, each object is only instantiated once all the objects before it have been loaded. For lots of objects, you can clearly see the loading happening.

The renderer service should support a request that processes a list of object spawns, not just a single one. On that request, the renderer can load each of the objects' assets without spawning them. If any of the asset loading fails, none of the objects are spawned and the service responds with an error. Otherwise, all of the objects are spawned into the world at once, and the list of capabilities to each of them are returned with a success response, with each entry in the list corresponding to the spawn request list entry.

[marceline-cramer]

@ten3roberts, if you're willing to, I think this issue would be great for you.