blur-effect: Properly clear background framebuffer

blur-effect: Properly clear background framebuffer

We want to completely clear the background framebuffer when switching
back to ACTOR mode to make sure the `background_fb.framebuffer` check
will fail in `update_background_fbo` when switching to BACKGROUND mode
again. Otherwise the checks in `update_background_fbo` will return TRUE
and we will keep using the background framebuffer that was created
before switchig to ACTOR mode.

While at it, also clear the background framebuffer completely when
changing the actor to avoid the same issue here.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1000