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.
2 jobs for night-light-always-on in 14 minutes and 9 seconds (queued for 99 minutes and 41 seconds)
latest
Status Job ID Name Coverage
  Build
passed #97636
build

00:04:53

 
  Test
passed #97637
test

00:09:15