Blender Git Loki

ColorManagement: Dithering Improvement

- Unlock property range.
- Use triangular noise to keep perceptual noise error more uniform.
Remap range to preserve perceptual intensity.
- Center noise distribution around 0 for GPU implementation because of
rounding.
- Do dithering after merging overlays.

Effect of using triangular noise is not really noticeable if you don't use
really low bitdepth. But doing a test in the shader were we artificially
reduce the bitdepth (`col = (col * 16) / 16;`) reveals the real difference.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6850