Background does not get repainted when moving windows with subsurfaces outside of main surface

I just encountered a case where we do not correctly draw the background anymore (apps are not affected). I run into it when tinkering with subsurfaces for !684 (merged).

When moving the window from the image above around, it leaves glitches over the background, but not over other apps. @carlosg, I haven't really tested it but it could have to do with !698 (merged)