Improve performance for large cities

[kfarr]

Cities with ~100+ objects seem to be slow in VR mode in FF nightly / Vive. There can be a noticeable delay when looking around or moving controllers which is nauseating and annoying. Here are some notes on optimization from a-frame slack conversation:

• enable a-frame stats - done, and noticed a few things. the control-bar component creates a new draw call for each object previewed, but this is not removed. so over time in a scene when objects are browsed this slows down the scene little by little.
• city builder users obj files. at the moment, each obj uses a separate mtl and png, but after some quick analysis those are usually identical (or very similar) and it's possible to load one mtl and use that same mtl reference for separate obj files. this may result in significant performance improvements and is probably the first place to try.
• research instancing of identical objects - https://github.com/takahirox/aframe-instancing/
• using "pool" is probably not going to offer significant performance improvement for this use case per slack conversation