Return a hex string for dynamic and grayscale colors in toHex

[Jeehut]

Overview

Color.toHex(alpha:) read cgColor, which is nil for dynamic colors (.primary, .blue, .systemBlue, …), so it returned nil for them. For colors in a gray color space it also returned nil (two components), and Display P3 colors were reported with their raw P3 numbers. It now reads sRGB components through UIColor/NSColor, which resolve dynamic colors, and clamps wide-gamut values to sRGB.

Color (macOS, dark)	main	This branch
.blue	nil	0091FF
.red / .gray / .primary	nil	FF4245 / 98989D / FFFFFF
CGColor(gray: 0.5)	nil	929292
Display P3 (0.5, 0.8, 0.3)	80CC4D (raw P3)	64CE2F (sRGB)
sRGB, white, black	unchanged	unchanged

Dynamic colors resolve against the traits current at the time of the call. The doc comment says so and points to rgbo(in:) for the color a view actually displays.

Split out of #7.

How this was verified

Red, with main's ColorExt.swift:

✘ hexOfDynamicColors(): color.toHex() is nil
✘ hexOfDisplayP3ColorIsConvertedToSRGB(): toHex() == "64CE2F"
✘ hexOfGrayscaleColor(): gray.toHex() is nil
✔ hexRoundTrips(), hexOfWideGamutColorIsClampedToSRGB()
✘ Test run with 5 tests in 1 suite failed with 3 issues.

Green, this branch:

✔ Test run with 5 tests in 1 suite passed.