Describe the bug
Follow up to #16323
Using a background option in ol/layer/Tile works because all the supplied options are passed to super() but this may cause problems for typescript users as it is not documented as an option. It does not work in ol/layer/WebGLTile.

Expected behavior
If background is to be a substitute for the tile source opaque option it should be documented and supported for both ol/layer/Tile and ol/layer/WebGLTile.