Support for 64-Bit Color Depth

[slavakurilyak]

Issue or Feature

It would be great to see support for 64-bit color depth (or bit depth).

Steps to Reproduce

None

Your Environment

None

[zbjornson]

Standard support for high bit-depth Canvas is still in early design stages: https://github.com/w3c/ColorWeb-CG. However, even if/when it reaches a more mature stage, it's unfortunately not something we can implement with the Cairo backend. Skia supports 16-bit floats per color component, were we to switch.