Copper: Can't round trip RGB/HSL for nvgHSL

[Paul-Dempsey]

nanovg provides nvgHSL* functions, but doesn't provide functions to extract the same h, s, or l from an NVGcolor.
I've written some basic functions that have been workable for the purposes of imagine, but it's a real problem for Copper, and I disabled hex editing.

To fix this the options are:

• reverse-engineering the math in nvgHSL*
• write an alternative color representation with consistent round-trip API, perhaps mining code from elsewhere