improve caching of resources (f.e. re-use textures from the same URL across elements instead of making one per element)

[trusktr]

Currently all resources are on a per-element basis. We should be smarter, and cache resources from the same URL when we can. For example,

• for any textures, we only need one for a particular URL, and it can be shared with all elements that use the same URL.
• for model loaders, we can load a model once, then perhaps clone it per element (because elements may want to modify the loaded model objects)
• etc

What about an escape hatch? What if someone wants to modify a loaded texture by reaching into the underlying .three object, and manually change some texture pixels?