effect json should only be loaded once per effect

[RandomEtc]

And preferably in a pre-loader, if we can.

It would be useful to be able to create an effect from either a path (as it is now), or a string, or a Json::Value. Using the latter would mean we can parse the cached data and quickly instantiate new effects.

Another option would be to implement a clone method on effects and cache the parsed and ready-to-go effect object itself. But caching raw data or parsed json seems easier to begin with.