backend: Do not use CLUTTER_WINDOWING_EGL unconditionally
backend: Do not use CLUTTER_WINDOWING_EGL unconditionally

https://bugzilla.gnome.org/show_bug.cgi?id=708079

Distrotech/clutter,ebassi/clutter,jigpu/clutter,jigpu/clutter,djdeath/clutter,djdeath/clutter,Distrotech/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter,ebassi/clutter,djdeath/clutter,jigpu/clutter,Distrotech/clutter,ebassi/clutter,GNOME/clutter,Distrotech/clutter,djdeath/clutter,jigpu/clutter,GNOME/clutter,GNOME/clutter,ebassi/clutter,Distrotech/clutter,jigpu/clutter,Distrotech/clutter,ebassi/clutter,GNOME/clutter,GNOME/clutter