lightbox: Do not assume GLSL is available

Some hardware does not support it causing crashes in cogl during paint.

https://bugzilla.gnome.org/show_bug.cgi?id=733623
st-scroll-view-fade: Refuse to work without GLSL

Disable the effect when GLSL is not available otherwise we will
crash later.

https://bugzilla.gnome.org/show_bug.cgi?id=733623