screenshot: Add preview to color picker

screenshot: Add preview to color picker

With color picking implemented in the compositor, we
can do better than letting the user pick a pixel with
the crosshair cursor, and present them with a preview
of the color that will be selected.

Do this by replacing the cursor with a custom icon and
apply a recoloring effect, where we replace a given color
with the color of the currently hovered pixel (similar
to a green screen).

https://gitlab.gnome.org/GNOME/gnome-shell/issues/451