Viewer: add clear-color attribute

[ryantrem]

This change adds a clear-color attribute to the Viewer html element. Since it is in the html layer, it relies on WebAPI to convert an arbitrary color string to a Babylon Color4. It is transparent by default. I also made one other small change to have the root div (:host in css) inherit all from the outer babylon-viewer element. I think this should be safe, and it lets you do things like this in CSS:

babylon-viewer {
  background-image: url("https://raw.githubusercontent.com/BabylonJS/Assets/master/textures/Checker_albedo.png");
}

Combining these two things together, with the default transparent clear-color you can get this:

[bjsplat]

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[bjsplat]

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