meta-background: Don't use brigthness twice

Until now the brigthness was being applied both to
the background and to the vignette effect. That was causing
the effect to not be the same as ligthbox in gnome-shell.
Since we are only using meta-background in the overview of
gnome-shell, we can delete the brigthness that was being applied
to the background and use only the brigthness applied to the
vignette effect, so we can match gnome-shell ligthbox vignette
effect.

https://bugzilla.gnome.org/show_bug.cgi?id=700081

[endlessm/eos-shell#3758]
background: 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

[endlessm/eos-shell#3758]
Merge pull request #20 from endlessm/shell/3758

Shell/3758