Render multiple passes

Related: https://github.com/chunky-dev/chunky/issues/18

Following a discussion on Discord.

A technique that has been around for a few years and detailed on [my guide](https://jackjt8.github.io/ChunkyGuide/) is [Split Pass](https://jackjt8.github.io/ChunkyGuide/docs/advanced_techniques/denoising.html#split-pass-denoising).

The principle concept is to take a scene and to separate it our into different passes which consist of the contributions of different light sources to the final product. These passes are then combined together in post to reproduce an image which is (almost) reference.
Each pass can have differing settings like Ray Depth and SPP which can aid in boosting rendering performance.

To setup a pass it was suggested to have a number of pre-set JSON files which could be used.
```
{
"name": "Sun, Sky, Fog Pass",
"emittersEnabled": false,
"sunEnabled": true,
"fogDensity": 0.1,
"sky": {
"mode": "BLACK"
}
}
```
and
```
{
"name": "Emitter Pass",
"emittersEnabled": true,
"sunEnabled": false,
"fogDensity": 0.0,
"sky": {
"mode": "SIMULATED"
}
}
```

Optionally we could even auto create GIMP project files to handle the default layering to recombine all the layers.