[loaders] Add async-texture system.

[donmccurdy]

I'd like to add a system or scene-level component that would automatically use ImageBitmapLoader (when supported) to decode images after the user enters VR mode. From previous results, this could reduce the number of dropped frames during texture upload. In the future we can consider alternative strategies (e.g. WebGL2) for non-blocking texture upload.

[donmccurdy]

Suggested API:

• scene-level component, async-textures
• use THREE.Loader.Handlers.add(regex, loader) to specify ImageBitmapLoader be used for PNG and JPEG files, for browsers that support it. (https://github.com/mrdoob/three.js/blob/master/src/loaders/Loader.js#L29)
• May need to be Chrome only, as the FF and Safari implementations had issues as of Nov 2017.