ocioview: Chromaticity inspector

[michdolan]

ocioview currently has inspectors for curves (#1845 ) and color volumes ( #1875 ).

This task would involved building a QGraphicsScene or OpenGL viewport which displays an interactive CIE chromaticity diagram, and plots the primaries of the currently viewed transform (if known) as a triangle, with the option to scatter plot all or some of the current image's transformed pixels into their xy coordinates over the diagram.