Add colourspace support

P3 displays are rather common now.
Add a setting to set the colourspace of canvas contexts.

https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/getContext#colorspace

https://webkit.org/blog/12058/wide-gamut-2d-graphics-using-html-canvas/

https://ccameron-chromium.github.io/webgl-examples/p3.html