Support rec2100-linear?

[svgeesus]

I wonder if we should support rec2100-linear from CSS Color HDR. It was requested by the Color on the Web CG, primarily for Canvas, and adopted by CSSWG resolution.

Although it is an HDR space, it is defined with white === color(rec2100-linear 1 1 1) (so the actual HDR values are above 1). That makes it an exact alias of color(--rec2020-linear), because BT.2100 uses the BT.2020 primaries and white point, differing only in the transfer function.

[LeaVerou]

I think we should generally aim to support a linear version of every RGB color space.