[css-color] Converting Between Uncalibrated CMYK and RGB-Based Colors

[svgeesus]

https://drafts.csswg.org/css-color/#cmyk-rgb

This section now needs to clearly distinguish between calibrated (icc-based) color on the one hand, and uncalibrated device-cmyk on the other. This particularly affects conversion to and from RGB.

Resolution depends on #301

[cabanier]

The PDF reference manual [1] sections 6.2.3 and 6.2.4 define a different conversion based on BlackGeneration and UnderColorRemoval.
Since those seem a bit easier(ie no divisions), maybe we can adopt them

1: www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdf_reference_1-7.pdf

[svgeesus]

The spec section cited says that BG and UCR are not used:
(sorry for the inaccessible screenshot, the PDF spec disallows copying text!)

However, agree; these simple conversions are commonly cited, they are not specific to PDF; and the spec now uses them.