Histogram and grayscale mode to viewport

Viewport histogram: a great help for adjusting the scene lighting and exposure/gamma to proper levels already in viewport render. Goal: make better decisions with shading/lighting and have less problems in post production.

Grayscale option in color management: a way to view a scene in grayscale, making it easier to adjust the color values of materials to proper levels. This helps to get the right balance between materials and helps to avoid unnatural color values. It also helps when trying to match a render to a reference image.