When using python offscreen rendering the background was not drawn in
EEVEE. Users would expect that when calling the `draw_view3d` method
that it shows the same as in the 3d viewport.
Seems to be a difference between the meaning of draw_background in b279
and b280. In B279 the world background would be drawn. In B280 the
transparency is changed.
Reviewed By: campbellbarton, fclem
Maniphest Tasks: T61768
Differential Revision: https://developer.blender.org/D5022
3c4a16acc5