extensions/notebook-renderers/src/color.ts/Color/makeOpaque
class Color:	makeOpaque(opaqueBackground: Color): Color {
		if (this.isOpaque() || opaqueBackground.rgba.a !== 1) {
			// only allow to blend onto a non-opaque color onto a opaque color
			return this;
		}

		const { r, g, b, a } = this.rgba;

		// https://stackoverflow.com/questions/12228548/finding-equivalent-color-with-opacity
		return new Color(new RGBA(
			opaqueBackground.rgba.r - a * (opaqueBackground.rgba.r - r),
			opaqueBackground.rgba.g - a * (opaqueBackground.rgba.g - g),
			opaqueBackground.rgba.b - a * (opaqueBackground.rgba.b - b),
			1
		));
	}