Allow explicit background color for 3.1 Canvas and 4.0 Scene

[azaroth42]

Clients seem to default currently to a black background for a canvas, but there's no reason for that to be the case. For a dark image, a white background might be more appropriate.

Proposal:

backgroundColor

The color to render as the background for a Canvas or Scene.

The value of the backgroundColor property MUST be a string representing a color according to the color specification for the CSS standard.

{
  "backgroundColor": "#FF00FF"
}

To discuss as well -- should a background Canvas be possible, to allow for images (e.g. checkerboard) or even video?

[azaroth42]

Rob and Dawn estimate: easy but should wait until we have the spec reorganized to write it.