Add a grayscale effect

This commit adds a grayscale effect to the magnifier, similar to
the lightness, brightness and contrast effects that are already there.

The effect is configured with the
org.gnome.desktop.a11y.magnifier.color-saturation setting, which
can take values from 0.0 (grayscale) to 1.0 (full color).

Based on a patch by Matthias Clasen <mclasen@redhat.com>

https://bugzilla.gnome.org/show_bug.cgi?id=676782