Add UE4 filmic tone mapping and make Hable tone mapping configurable.

[leMaik]

I just wanted to render a few example images for the post-processing section of the docs, and somehow went down this rabbit hole.

1. UE4 Filmic Tonemapping is now implemented; basically configurable ACES, defaults to matching the ACES curve (UE4 legacy tone mapping preset is also in the code already)
2. Hable was wrong (missing Gamma Correction and arbitrary x16 exposure)
3. Compared Hable's GDC talk with the blog post and noticed that there were different parameters; added presets for both
4. Hable and UE4 Filmic Tonemaps can now be configured (JSON only until someone adds a UI)