Support wide-gamut colour spaces

[benjaminleonard]

uPlot creates its drawing context with no attributes:

const ctx = self.ctx = can.getContext("2d")

which means the canvas is always sRGB. Colours that exceed the sRGB gamut get clamped when drawn to the canvas, so on wide-gamut (P3) displays a uPlot chart looks visibly washed out next to the exact same colour rendered by CSS/SVG elsewhere on the page.

// example attrs
{ ctxAttrs: { colorSpace: 'display-p3' } 

const ctx = self.ctx = can.getContext("2d", opts.ctxAttrs);

I think browsers that don't support colorSpace ignore it, so no feature detection is needed in uPlot itself. desynchronized and alpha (in the context attrs) are likely not useful, so an alternative is passing colorSpace as a bespoke option.

Happy to submit a PR if you're open to it.