new simple shortcut for layout.mapbox.style: white-bg #4051

I propose another new shortcut value for layout.mapbox.style called "white-bg" with the following definition:

{
        "id": "white-bg",
        "version": 8,
        "sources": {},
        "layers": [
          {
            "id": "white-bg",
            "type": "background",
            "paint": {"background-color": "#FFFFFF"}
          }
        ]
      }

This would be useful in conjunction with #4050 when a user wants a very simple way to provide a layer to go under their data. They would provide layout.mapbox.style: "white-bg" and then some data and then something in layout.mapbox.layers with below: traces. If their layer is fully opaque (i.e. some raster tile-server) then the white background won't be visible, but it also won't make any API calls to any server.