Re: master 68ae6fa: Improved light/dark colour predicate (bug#41544)

[Mattias Engdegård]

Thanks to those who replied -- the originally proposed 0.325 luminance limit 
seems to be fairly uncontroversial and can be changed if evidence indicates 
that it was badly chosen. Most of all, the value seems to work well across all 
platforms and a sufficient variety of equipment, which is the important part.

Special thanks to Yuri for helping me confirm that the theoretical 50% 
lightness of luminance 0.18 wasn't necessarily an optimal value in this case. 
Trust my own eyes, but verify!

The constant has now been given a (provisional) name, for extra clarity and 
ease of adjustment.