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).

Fixes #451 (closed)

(Depends on mutter!1244 (merged) at runtime)