Fix performance regression with only one active terminal widget.
Fix performance regression with only one active terminal widget.

https://bugzilla.gnome.org/show_bug.cgi?id=721944

flaxandteal/gasket-vte,rvu95/vte,saitoha/vte-sixel,rvu95/vte,flaxandteal/gasket-vte,saitoha/vte-sixel,pjm0616/vte,thestinger/vte-ng,jessevdk/vte,rvu95/vte,pjm0616/vte,pjm0616/vte,jessevdk/vte,thestinger/vte-ng,flaxandteal/gasket-vte,saitoha/vte-sixel,flaxandteal/gasket-vte,jessevdk/vte,jessevdk/vte,thestinger/vte-ng,rvu95/vte