CSS Color 4 color spaces for canvasGradient

[mysteryDate]

A draft PR for the changes discussed in these two issues:

#7947
#8296

What's described here is method 3 from #8296, as implementers seemed to think that mutating gradients was a fine approach, and this is certainly the least spec intensive way.

Chromium bug: crbug.com/394052224

Open questions:

• Should we add an "options" object at the end, to specify these attributes at creation? (Method 2 in Define color space for interpolation interface for canvas gradients #8296)
• Do we expose color stops, while we're at it?

TODO:

• Lint
• Mozilla/Apple stances, bugs
• Experimental implementation in Chromium
• WPT tests

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/canvas.html ( diff )