color: Allow night light to be always on

Currently if the user configures the "from" and "to" times for the night light schedule to be close together or equal, g-s-d gets confused. It tries to smear the transition from cool to warm light and vice versa over the course of an hour, but if the two times are within an hour of each other the night light ends up being disabled except for a period around the two times.

This commit disables smearing if the times are too close together for it to make sense, so users can effectively make night light "always on" or almost always on if they want. GNOME Control Center will have to be updated as well to display an accurate depiction of the new semantics.