Adam Barth 49b183c502 Estimate the brightness of the primary color (#10071)
If the caller doesn't explicitly give the brightness of the primary
color, we now estimate it using an algorithm from the Web Content
Accessibility Guidelines.

Also, this patch contains a function that converts RGB colors to
HSVColors. I was originally going to use that, but the WCAG algorithm
ended up seeming like a better choice. The patch still includes this
function because it's generally useful.

Fixes #5718
2017-05-14 10:09:52 -07:00
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