[css-color] wider/deeper colors

In preparation for discussion of this topic at the face-to-face meeting, a summary of where (I think) we are on this topic:

0. Firstly, not much has changed since my last email thread on this topic, which has some good discussion [1].

1. The specification already allows values outside of [0,1] inside the rgb() function, although no-one implements it yet. However, we don't describe what such values actually mean. I think we should investigate describing them as using the scRGB space [2].

2. We will add a new function for describing colors that accepts a color-profile name and a variable number of arguments. e.g. color("bt2020", 0.7, 0.3, 0.1). The name can be linked to a @color-profile, but we will also have some predefined keywords for the most common profiles.

2.a (alternate) Similar to 2 but just add a function that accepts BT.2020 because "640kb ought to be enough for anybody".

3. Add a media query to enable detection of a hardware and software stack that will process colors outside of sRGB. This could take the form of "awesome-colors: none | some | plenty" which map to what most people have today, something about DCI P3, and then something about BT.2020 (i.e intentionally slightly vague), or something that allows detection of a named profile.

3.a (alternate) Florian suggested something more like @supports, where you give something from points 1 or 2 above, and the system will tell you if it could handle it. I like this idea, but we'd need to be careful to describe what it means to handle the color. 

[1] https://lists.w3.org/Archives/Public/www-style/2015Nov/0017.html
[2] https://en.wikipedia.org/wiki/ScRGB <https://en.wikipedia.org/wiki/ScRGB>

Dean