Props to @samirosman for spotting the typo in the comment
describing the proper luma / luminance weighting coefficient
for the blue REC.709 primary. Updated the other slippery "RGB"
terms to more properly reflect the assumptions.
While still not colour managed, and grossly assuming REC.709
display referred buffers, the effect as it currently stands can be
improved to use the proper coefficients for REC.709, as opposed
to legacy REC.601. Ideally there'd be a flag somewhere to flip between
the two coefficient sets.
Troy James Sobotka