Initial implementation of a reader for TexturePacker JSON sprite sheet files
Try to use a sprite sheet for the render test (it's not working for now due to the way webworkers work out their base URL)
Use some fake scripts to get the Base URL set to the right place in the worker script. It's quite important that it's consistent between the worker and the main page.
Use the path prefix for the JSON asset to calculate the location of the png files as well (code is a giant hack due to lack of URI parsing support in JavaScript)