fix bug stuck backgrounds
fix bug stuck backgrounds

Only deal with opacity of background in the transition specific code
when there is a transition. This avoids an issue where sometimes the
background used in a transition was also visible for other slides.

nekohayo/pinpoint,danigm/pinpoint,bpeel/pinpoint,potofcoffee/pinpoint,danigm/pinpoint,robclark/pinpoint,potofcoffee/pinpoint,bpeel/pinpoint,nekohayo/pinpoint,GNOME/pinpoint,nzjrs/pinpoint,GNOME/pinpoint,michaelgwood/pinpoint,robclark/pinpoint,nzjrs/pinpoint,robclark/pinpoint,tyll/pinpoint,tyll/pinpoint,michaelgwood/pinpoint