Niklas Haas authored Dec 28, 2017 and Thomas Guillem committed Jan 04, 2018

This allows users to override the target colorspace, some of the
colorspace conversion options, the tone mapping settings, and some of
the dithering settings.

Settings which I decided to omit include:
- dither LUT size (limited by the shader embedding method)
- temporal dither (not currently possible)
- overrides for the source colorspace (shouldn't persist)
- target display's light type (unlikely to not be _DISPLAY)
- target display's HDR metadata (usually taken care of by the display)

Signed-off-by: Thomas Guillem <thomas@gllm.fr>